Cone Idioblasts of Eleven Cycad Genera: Morphology, Distribution, andSignificance

Andrew P. Vovides

Botanical Gazette, Vol. 152, No. 1. (Mar., 1991), pp. 91-99.

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BOT. GAZ. 152(1):91-99. 1991. 0 1991 by The University of Chicago. All rights reserved. 0006-8071 /91/5201-0012$02.00

CONE IDIOBLASTS OF ELEVEN CYCAD GENERA: MORPHOLOGY,

DISTRIBUTION, AND SIGNIFICANCE

ANDREW P. VOVIDES

Fairchild Tropical Garden, 11935 Old Cutler Road, Miami, Florida 33156; and Instituto de Ecologia, A.C., Apdo Postal 63, Xalapa, Veracruz 91000, Mexico

Sporophylls from strobili of 16 species among 11 genera of cycads were examined for idioblasts. Thin- walled secretory-like idioblasts were found in the majority of taxa and thick-walled sclerenchyma-type idio- blasts were found in the minority. With the exception of Cycas rumphii and Stangeria eriopus, where secretory idioblasts were in the epidermis and/or hypodermis, none were found in the sporophyll paren- chyma of these species. Most idioblasts stained ninhydrin-Schiff-positive, and tannins were present also. Sporophyll idioblasts appear to be related to interactions with insect predators and/or cosymbionts and may form part of a complex pollination syndrome. The lack of idioblasts in stem tissue and low concentration in leaflet tissue of Zamiafutfiuracea compared with sporophyll tissue is significant support to this hypothesis. On the basis of almost universal occurrence of these idioblasts in the sporophylls, we suggest that insect symbiosis related to pollination may be common to most cycad genera.

Introduction

Idioblasts may be classified into three major cat- egories: secretory, tracheoid, and sclerenchyma- tous (FOSTER 1956). The term "idioblast" may be applied to a range of cell types from parenchyma cells with specialized contents like tannin cells and oil cells to scelerenchymatous cells, idioblastic sclereids, or trichoblasts that are distributed ran- domly in the soft tissues of organs for which they provide support (METCALFE 1979). The and CHALK presence of idioblasts in cycads has been noted for some time (GREGUSS 1968), but little is known of their function. NORSTOG and FAWCETT (1989) and NORSTOGand VOVIDES (unpubl. data) relate the idioblasts from staminate and ovulate cones of the Mexican cycad Zamia furfuracea to a complex weevil-pollination syndrome. The idioblasts stain ninhydrin-Schiff (NIN) and periodic acid-Schiff (PAS) positive (NORSTOG and FAWCETT 1989). Re- cent analysis of cycad tissues by DUNCAN et al. (in press) has shown the presence of a powerful neu- rotoxin 2-amino-3-(methylamino)-propanoic acid (BMAA). This toxin is sequestered in the larval phase of the weevil pollinator and incorporated into the pupa case (NORSTOG, DUNCAN,and FAWCETT in preparation). DUNCAN et al. (in press) and NOR- STOG et al. (1986) suggest that these toxins are con- tained within idioblasts in the parenchyma tissues of the staminate and ovulate cone sporophylls of Z. fufuracea. Cycads are also toxic to most mam- mals including humans (WHITING 1963, 1965).

Another ergastic substance present in cycads is calcium oxalate (CWERLAIN 1919) usually in the

Manuscript received April 1990; revised manuscript received September 1990.

Address for correspondence and reprints: ANDREW P. VOV- IDES,Instituto de Ecologia, A.C., Apdo Postal 63, Xalapa, Ve- racruz 9 1000, Mexico.

form of crystals or druses. Though very wide-spread in angiosperms and much studied by ana-tomists there are differences of opinion as to the significance of calcium oxalate crystals in metab- olism (METCALFE and CHALK 1983).

This investigation is a survey of the cones of 16 species among 11 genera of cycads growing at Fairchild Tropical Garden (FTG), undertaken to describe idioblasts within their sporophyll tissues in comparison to those found in the sporophyll tis- sues of Z. furfuracea.

Material and methods

Micro- and megasporophylls from early and near maturity cones were fixed in formalin-acetic acid-alcohol (FAA) then embedded in wax for micro- tome sectioning. Cross and longitudinal sections, 20 pm thick, were taken and duplicate slides were prepared for histochemical staining using NIN (proteins), PAS (carbohydrates), phloroglucinol-HC1 (lignin), iodine (starch), and ferrous sulfate (tan- nins). Crossed polaroids were used on the micro- scope to detect birefringent crystals, and calcium oxalate was tested for with concentrated HCl (METCALFE 1983). Staining and mount- and CHALK ing procedures were according to JENSEN (1962) and JOHANSEN (1940). For Bowenia and Microcy-cas, herbarium specimens were hydrated by boil- ing in water for 15 min and cooling (repeated three times) before fixing and dehydrating as with fresh material.

Cross sections of leaflet and stem of Zamia fur- furacea and leaflet of Encephalartos altensteinii were taken for comparison with the cone tissue.

A Leitz Ortholux 2 microscope was used for photomicrography with Kodak Plus-X film (125 ASA) and Kodabrome RC paper for prints. The taxa examined are listed with their FTG accession no. or collectors' number for the herbarium ma-terial examined at FTG (table 1).

92 BOTANICAL GAZETTE

TABLE I

Species Garden accession no. Voucher no.

Bovvenia spectabilis Hook. ex Hook. f . . . . . . . . . . Ceratozamia mexicana Brongn. . . . . . . . . . . . . . . . . C . mexicana var. robusta Miq. . . . . . . . . . . . . . . . . Cycas rumphii Miq. . . . . . . . . . . . . . . . . . . . . . . . . . Chigua restrepoi Stevenson . . . . . . . . . . . . . . . . . . . Dioon edule Lindl. . . . . . . . . . . . . . . . . . . . . . . . . . . D. spinulosum Dyer . . . . . . . . . . . . . . . . . . . . . . . . . Encephalartos bubalinus Melville . . . . . . . . . . . . . . E. ferox Bertolini . . . . . . . . . . . . . . . . . . . . . . . . . . . E. hildebrandtii. A. Braun et Bouche . . . . . . . . . . . E. manikensis (Gilliland) Gilliland. . . . . . . . . . . . . . Lepidozamia hopei Regel . . . . . . . . . . . . . . . . . . . . . Macrozamia lucida L.A.S. Johnson . . . . . . . . . . . . Microcycas calocoma (Miq.) D.C. . . . . . . . . . . . . . Stangeria eriopus (Kuntze) Nasn . . . . . . . . . . . . . . . Zamia fischeri Miq. . . . . . . . . . . . . . . . . . . . . . . . . 2. lindenii Regel ex Andre. . . . . . . . . . . . . . . . . . . .

Observations contained secretory, parenchymatous idioblasts and All genera examined had at least secretory idio- oblong, angular to isodiametric, thick-walled,

blasts throughout the parenchyma of the sporophyll idioblastic sclereids that were NIN-positive and tissues (figs. 1-20). Exceptions to these observa- PAS-negative. Epidermal cells stained NIN-posi- tions were Cycas and Stangeria, which had few tive. Megasporophylls were not examined. Most idioblasts that were confined to the epidermal and/ idioblasts and epidermal cells stained for tannins. or hypodermal cells. All idioblasts were found to Idioblast break-down was not detected. be NIN-positive except those of Cycas rumphii, Ceratozamia mexicana (fig. 2), in microsporo- Chigua restrepoi, Microcycas calocoma, and Stan- phylls, idioblasts were more concentrated in the geria eriopus. All trichoblasts and idioblastic hypodermis, and the trichoblasts were mostly PAS sclereids had lignified thick secondary cell walls, negative. Much starch was present throughout the the trichoblasts were 7-15 times longer than wide parenchyma with very few idioblasts breaking down. and were often branched at their ends (figs. 6, 7). Megasporophylls of C . mexicana var. robusta (im-The idioblastic sclereids were oblong, angular, or mature cone) had both parenchymatous, secretory isodiametric resembling stone-cells but each with idioblasts, and septate trichoblasts that were NIN- a large lumen. Most showed heavy pitting in their and PAS-positive. Their idioblasts and epidermal thick secondary walls (figs. 10, 12). Most species cells also contained tannins. Calcium oxalate druses showed crystals and druses in their tissues, and some were found in cells near the epidermis and very Encephalartos species showed druses associated little starch was present in the parenchyma. Some with their cuticles. of the secretory idioblasts have transfer cell wall-

Bowenia spectabilis (fig. 1) microsporophylls like structures but no evidence of breakdown.

FIGS. 1-10.-Micro- and megasporophylls in section. Fig. 1, Bovvenia spectabilis cross section of microsporophyll, idioblasts (arrows), iodine-fast green (IFG); insert a=ninhydrin-Schiff (NIN) stain. Fig. 2, Ceratozamia mexicana cross section of micro- sporophyll, secretory idioblasts (arrow) NIN stain; insert a = C . mexicana var. robusta cross section of megasporophyll, trichoblast (large arrow), secretory transfer cell-like idioblast (small arrow), NIN stain; insert b=microsporophyll parenchyma cells packed with starch, IFG stain. Fig. 3, Chigua restrepoi cross section of megasporophyll, secretory idioblasts (arrows); insert a=transfer cell-like secretory idioblast, ferrous sulfate stain for tannins (black). Fig. 4, Cycas rumphii cross section of micro- and mega- sporophylls (insert a ) idioblasts and tannin-filled epidermal cells (arrows). Figs. 5, 6, Dioon edule. Fig. 5, Cross section of microsporophyll, idioblasts (arrows) NIN stain; insert a=starch-filled parenchyma cells, IFG stain. Fig. 6, Longitudinal section of megasporophyll with trichoblasts (large arrow) and secretory idioblasts (small arrow) NIN stain; insert a=tannin-filled epidermal cells, ferrous sulfate stain. Figs. 7, 8, Dioon spinulosum. Fig. 7, Longitudinal section of microsporophyll, NIN-positive secretory idioblasts (arrows) and trichoblast; insert a=starch-filled parenchyma cells, IFG stain; insert b=tannin-filled idioblasts, ferrous sulfate stain. Fig. 8, Cross section of megasporophyll showing NIN-positive secretory idioblasts with some having transfer cell- like appearance (insert a) . Fig. 9, Encephalartos bubalinus, cross section of megasporophyll, NIN-positive idioblasts (arrows), transfer cell-like secretory idioblasts (small arrow). Fig. 10, Encephalartosferox, cross section of microsporophyll, NIN-positive secretory idioblasts and pitted idioblastic sclereids; insert a=starch-filled parenchyma cells, IFG stain. All bars 50 km.

95 VOVIDES-CYCAD CONE IDIOBLASTS

Chigua restrepoi (fig. 3 ) rnegasporophylls con-tained secretory, parenchymatous idioblasts near the epidermis that stained NIN-negative and mostly PAS-negative. Trichomes stained NIN-positive. Microsporophylls were not examined. Most idio- blasts and epidermal cells stained for tannins. Many idioblasts showed transfer cell wall-like structures and appeared to be breaking down. Few druses were found in sporophyll tissues associated with the epi- dermis. No starch was detected in the sporophyll parenchyma.

Cycas rumphii (fig. 4) micro- and megasporo- phylls contained a few secretory idioblasts in the first layers of cells beneath the epidermis (hypoder- mis) but not elsewhere. Microsporophylls also had thick-walled isodiametric, idioblastic sclereids. The epidermal cells and idioblasts were NIN- and PAS- negative and contained tannins, but trichomes were found to be NIN-positive. Druses were found in both micro- and megasporophyll tissues, especially near the epidermis, and some druses were scattered in the base of the microsporophyll stalk. Very little starch was detected in megasporophyll paren-chyma, but much was found in the microsporo- phyll tissue. The breakdown of idioblasts in the megasporophylls was not detected.

Dioon edule (figs. 5 , 6) micro- and megaspo- rophylls contained secretory, parenchymatous idioblasts and thick-walled, septate trichoblasts that were NIN- and PAS-positive. Some epidermal cells of the microsporophyll were NIN-positive, and all epidermal cells of the megasporophyll and tri-chomes were NIN-positive. Idioblasts and epider- mal cells of both micro- and rnegasporophylls were rich in tannins. No idioblast breakdown was de- tected in the megasporophyll (immature cone), but some secretory idioblasts of the rnicrosporophyll were seen to have a transfer cell wall-like struc-tures. Few druses were found in both sporophyll tissues. No starch was detected in the megaspo- rophylls, but much starch was found in the mi- crosporophyll parenchyma.

Dioon spinulosum (figs. 7 , 8) micro- and mega- sporophylls contained secretory, parenchymatous idioblasts and thick-walled septate trichoblasts that

were NIN- and PAS-positive in megasporophyll tissues and NIN-positive and mostly PAS-positive in microsporophylls. Epidermal cells of both mi- cro- and megasporophylls were NIN-positive. Most idioblasts in the microsporophyll, and all epider- mal cells of both micro- and megasporophylls, were rich in tannins. Almost all idioblasts showed trans- fer cell wall-like structures in the megasporophyll, but some secretory idioblasts of the microsporo- phyll were also seen to have transfer cell wall-like structures. Few druses were found in both sporo- phyll tissues. No starch was detected in the me- gasporophylls, but much starch was found in the microsporophyll parenchyma.

Encephalartos bubalinus (fig. 9 ) megasporo-phylls contained secretory, parenchymatous idio- blasts and oblong, angular to isodiametric, thick- walled, pitted, idioblastic sclereids that were NIN-positive and mostly PAS-negative. Micro-sporophylls were not examined. All idioblasts and epidermal cells stained for tannins. Few idio- blasts showed transfer cell wall-like structures and were breaking down. Druses were found in sporo- phyll tissues associated with the epidermis and cu- ticle. No starch was detected in the sporophyll parenchyma.

Encephalartos ferox (fig. 10) microsporophylls contained secretory, parenchymatous idioblasts and oblong, angular to isodiametric, thick-walled, pit- ted, idioblastic sclereids that were NIN-positive and mostly PAS-positive. Megasporophylls were not examined. Most idioblasts but no epidermal cells stained for tannins. Some idioblasts showed trans- fer cell wall-like structures and appeared to be breaking down. A few druses were found in spo- rophyll tissues associated with the epidermis and cuticle. Starch was detected in the sporophyll parenchyma.

Encephalartos hildebrandtii (fig. 1 1) micro- and rnegasporophylls contained secretory, parenchy-matous idioblasts and oblong, angular to isodi-ametric, thick-walled, pitted, idioblastic sclereids that were NIN- and PAS-positive in megasporo- phyll tissues and NIN-negative and mostly PAS- negative in the microsporophyll. Epidermal cells

FIGS. 1 I-20.-Micro- and megasporophylls in cross section. Fig. 1 1, Encephalartos hildebrandtii, microsporophyll with NIN- positive secretory idioblasts and idioblastic sclereids; insert a=megasporophyll with NIN-positive transfer cell-like secretory idio- blasts (arrow). Fig. 12, Encephalartos manikensis, microsporophyll with NIN-positive secretory idioblasts and large, pitted idio- blastic sclereids (insert a) . Fig. 13, Lepidozamia hopei, megasporophyll with tannin-filled secretory idioblasts near epidermis (small arrow) and transfer cell-like (large arrow); insert a=NIN-positive idioblasts. Fig. 14, Macrozamia lucida, megasporophyll with NIN-positive idioblasts; insert a=microsporophyll with NIN-positive idioblasts; insert b=microsporophyll with tannin-filled idioblasts, transfer-like cells (arrows). Fig. 15, Microcycas calocoma, megasporophyll; insert a=microsporophyll, tannin-filled idioblasts (arrows). Figs. 16, 17, Stangeria eriopus. Fig. 16, Microsporophyll with tannin-filled epidermal idioblasts (arrow); insert a=starch-filled parenchyma cells (arrow), IFG stain; insert b=megasporophyll, ferrous sulfate stain showing tannin in epi- dermal idioblasts. Fig. 17, Megasporophyll with mostly NIN-negative epidermal idioblasts (insert a); insert b shows NIN-positive epidermal glandular hair. Figs. 18, 19, Zamiafischeri. Fig. 18, Young microsporophyll with small (undeveloped) secretory idio- blasts (arrows) some of which are NIN-positive; insert a=microsporophyll at pollen release with large, fully developed, intact NIN-positive secretory idioblasts. Fig. 19, Megasporophyll with NIN-positive secretory idioblasts; insert a=transfer cell-like idioblast. Fig. 20, Zamia lindenii, megasporophyll with NIN-positive secretory idioblasts. All bars 50 km.

96 BOTANICAL GAZETTE [MARCH

of the megasporophyll only were NIN-positive. Most idioblasts and some epidermal cells of the mega- sporophyll and a few idioblasts of the microspo- rophyll were rich in tannins. Almost all idioblasts showed transfer cell wall-like structures and ap- peared to be breaking down in the megasporophyll, but only a few idioblasts of the microsporophyll were seen to show this phenomenon. A few druses were found in both sporophyll tissues associated with the epidermis. No starch was detected in the megasporophyll, but starch was found in the mi- crosporophyll parenchyma.

Encephalartos manikensis (fig. 12) microspo- rophylls contained secretory, parenchymatous idioblasts and oblong, angular to isodiametric, thick- walled, pitted, idioblastic sclereids that were NIN- and PAS-positive, epidermal cells stained NIN-positive. Megasporophylls were not examined. Some idioblasts showed transfer cell wall-like structures and evidence of breakdown. A few druses were found in sporophyll tissues associated with epider- mis and cuticle. Starch was detected in the spo- rophyll parenchyma.

Lepidozamia hopei (fig. 13) rnegasporophylls contained secretory, parenchymatous idioblasts concentrated near the epidermis that stained NIN- and PAS-positive; microsporophylls were not examined. All idioblasts and epidermal cells stained for tannins. Most idioblasts showed transfer cell wall-like structures and evidence of breakdown. Few druses were found in the sporophyll tissues. No starch was detected in the sporophyll parenchyma.

Macrozamia lucida (fig. 14) micro- and mega- sporophylls contained secretory, parenchymatous idioblasts and idioblastic sclereids that were NIN- and PAS-positive in rnicrosporophyll tissues and NIN-negative and mostly PAS-negative idioblastic sclereids in the megasporophyll. Epidermal cells of both micro- and megasporophylls were NIN-posi- tive as well as trichomes on megasporophylls. All idioblasts and epidermal cells of both micro- and megasporophylls were rich in tannins. Idioblasts of both micro- and megasporophylls showed transfer cell wall-like structures and evidence of break- down. Druses were found in rnicrosporophyll tis- sues associated with the epidermis. No starch was detected in the megasporophyll, but starch was found in the microsporophyll parenchyma.

Microcycas calocoma (fig. 15) micro- and mega- sporophylls contained secretory, parenchymatous idioblasts that stained NIN- and PAS-negative in rnicrosporophyll tissues and were NIN-positive and PAS-negative idioblastic sclereids in mega- sporophylls. Trichomes of both micro- and mega- sporophylls stained NIN- and PAS-positive. Most epidermal cells of megasporophylls stained NIN- positive, but few epidermal cells of the micro- sporophyll stained similarly. Most idioblasts of mi-

cro- and megasporophylls were rich in tannins. Idioblasts of microsporophylls showed transfer cell wall-like structures and evidence of breakdown; those of rnegasporophylls appeared crushed. Druses were found scattered throughout in microsporo-phyll tissues, but they were near the epidermis in megasporophylls.

Stangeria eriopus (figs. 16, 17) micro- and megasporophylls contained secretory, parenchy-matous idioblasts associated only with the epider- mis. Most of the idioblasts stained NIN- and PAS- negative in microsporophylls which also have NIN-positive thick-walled hypodermal cells, and they were NIN- and PAS-positive in rnegasporophylls. Trichomes and glandular hairs of both micro- and megasporophylls stained NIN- and PAS-positive. Few idioblasts of micro- and rnegasporophylls were rich in tannins. Idioblasts in the epidermis of megasporophylls showed transfer cell wall-like structures and evidence of breakdown. Druses were found scattered throughout in micro- and mega- sporophyll tissues. Starch was detected in micro- sporophyll parenchyma, but very little was present in megasporophyll tissue.

Zamia fischeri (figs. 18, 19) micro- and mega- sporophylls contained secretory, parenchymatous idioblasts mostly concentrated near the epidermis, most of which stained NIN- and PAS-positive in rnegasporophylls but PAS-negative in micro-sporophylls. Trichomes of both micro- and mega- sporophylls stained NIN- and PAS-positive. Idio- blasts of megasporophylls were rich in tannins. No tannins were detected in young microsporophyll idioblasts and little tannin was detected in idio- blasts of mature microsporophylls. Idioblasts of megasporophylls showed transfer cell wall-like structures and evidence of breakdown. Fqw druses were found scattered throughout in micro- and mega- sporophyll tissues. Much starch was detected in the parenchyma of mature microsporophylls, but there was none in the parenchyma of young microspo- rophylls. Idioblast size is notably smaller in young than in mature microsporophylls.

Zamia lindenii (fig. 20) megasporophylls con- tained secretory, parenchymatous idioblasts that stained NIN-positive and mostly PAS-negative; tri- chomes stained NIN-positive. Microsporophylls were not examined. Tannins were not present in any of the idioblasts or epidermal cells. No idio- blasts showed transfer cell wall-like structures or evidence of breakdown. Few druses were found in sporophyll tissues near the epidermis. No starch was detected in the sporophyll parenchyma.

The frequency of idioblasts in megasporophylls of a receptive ovulate cone of Zamia furfuracea ranges from 52 to 79 per mm2 of sporophyll tissue with a mean of 64 (N = 3). The stem tissue did not show any idioblasts; in leaflet tissue few NIN- positive idioblasts were seen associated with the

19911 97 VOVIDES-CYCAD

vascular bundles, ranging from 11 to 20 per mm2 of leaflet tissue with a mean of 15.7 (N = 3). No idioblasts were seen in E. altensteinii leaflet tissue.

Discussion

Idioblasts in plants, in general, vary widely in morphology and have presumedly different func- tions that generally have not been studied in detail. They may contain ergastic material such as oils, mucilage, tannins, phenolic compounds, and crys- tals (FOSTER 1956; ZOBEL 1986). The cycads stud- ied appear to have two types of idioblasts in their cones: secretory idioblasts and sclerenchymatous, the former to a greater extent. Secretory idioblasts appear to be similar in morphology throughout the genera, differing only in size. The sclerenchyma- tous idioblasts vary greatly among the genera rang- ing from thick-walled, trichoblast-type cells of Ceratozamia to the pitted, idioblastic sclereids of Encephalartos. Some of the secretory idioblasts, especially prior to and during breakdown, show transfer cell wall structures described by GUNNING and PATE (1969) and in Zamia furfuracea (NORSTOGand FAWCETT 1989). The latter authors considered this appearance to be evidence of idio-

CONE IDIOBLASTS

blast breakdown and toxin secretion. NORSTOG and FAWCETT (1989) and VOVIDES, NORSTOG,and DUNCAN(unpublished data) have recently drawn attention to the function of these cells in cycads in relation to an insect pollination syndrome in Z . furfuracea. Beetle pollination of Z . furfur- acea, Z. pumila, and probably other cycads, is linked with a food reward (starch) as well as brood place and shelter in the staminate cones. Such re- wards are not present in ovulate cones of Zamia, but nevertheless, pollinating beetles visit them briefly thereby completing pollination (NORSTOG et al. 1986; NORSTOG 1987; TANG 1987; NORSTOG and FAWCETT 1989). What attracts the insect to the ovulate cones is not known, although fragrances may act as attractants (TANG unpublished data). More important, idioblasts of Z . furfuracea have been found to contain 2-amino-3-(methylamino)- propanoic acid (BMAA) (DUNCAN, personal com- munication) and may also contain other toxins and function differentially in staminate cones and ovu- late cone sporophylls. The presence of tannins may be associated with phenolic compounds (ZOBEL 1986). The presence of NIN-positive sporophyll idioblasts in both sexes seems to be significant in that idioblasts are generally intact in staminate cones,

TABLE 2

SUMMARYOF IDIOBLAST DATA AND OTHER CELL CONTENTS OF SPOROPHYLLS OF

18 TAXA AMONG l l CYCAD GENERA

Idioblast transfer cell/

Species Sex breakdown" Starch Ninhydrin

. . . . . . . . . . . . . .Bowenia spectabilis M . . . . . . . . . . . .Ceratozamia mexicana M

. . . . . . . . .C . mexicana var. robusta. F Chigua restrepoi . . . . . . . . . . . . . . . . . F Cycas rumphii.. . . . . . . . . . . . . . . . . . M C . rumphii.. . . . . . . . . . . . . . . . . . . . . F Dioon edule . . . . . . . . . . . . . . . . . . . . . M

. . . . . . . . . . . . . . . . . . . . . . .D. edule F . . . . . . . . . . . . . . . . .D. spinulosum.. M

D. spinulosum.. . . . . . . . . . . . . . . . . . F . . . . . . . . . .Encephalartos bubalinus F

E . ferox . . . . . . . . . . . . . . . . . . . . . . . . M E. hildebrandtii.. . . . . . . . . . . . . . . . . M E. hildebrandtii. . . . . . . . . . . . . . . . . . F

. . . . . . . . . . . . . . . . . . .E . manikensis M Lepidozamia hopei . . . . . . . . . . . . . . . F Macrozamia lucida . . . . . . . . . . . . . . . M

. . . . . . . . . . . . . . . . . . . . .M . lucida.. F . . . . . . . . . . . . .Microcycas calocoma M

. . .. . . . . . . . . . . . . .M. calocoma .. F . . . . . . . . . . . . . . . .Stangeria eriopus M

. . . . . . . . . . . . . . . . . . . . . .S. eriopus F . . . . . . . . . . . . . . . . . . .Zumia fischeri M

. . . . . . . . . . . . . . . . . . . . . .Z.fischeri F

. . . . . . . . . . . . . . . . . . . . . .Z . lindenii F

"here some or few idioblasts show transfer cell-like structures or breakdown in the text, it is considered negative in this summary.

98 BOTANICAL GAZETTE [MARCH

but in ovulate cones they break down probably re- leasing toxin just before receptivity to pollen (NOR-STOG and FAWCETT1989). Because idioblasts of staminate cones do not break down and since the life cycle of the insect is completed in the stami- nate cones, it is thought that the function of stam- inate-cone idioblasts is to sequester toxin. Further, the larvae make their pupa cases from their excre- ment, and these are found to be rich in the cycad neurotoxin, BMAA. The BMAA thus incorporated in the pupa case may serve the larva as protection from predation during metamorphosis (NORSTOG, DUNCAN, and FAWCETT,unpublished data).

The presence of idioblasts in sporophylls of both sexes and the predominance of starch in the mi- crosporophylls of the taxa studied is assumed to be related to interactions of one kind or another with insect predators and/or cosymbionts giving rise to a similar pollination syndrome described for Z. furjiuracea by NORSTOGand FAWCETT(1989). Although insects related to cones of other cycads remain unidentified to species, weevils in the ge- nus Rhopalotria and Languriid beetles have been isolated from cones of Dioon edule, D. spinu-losum, Ceratozamia mexicana, and D. califanoi, all collected from natural habitats in Mexico (VOV-IDES, in press).

A different pollination syndrome from that of Z. furfuracea may be found in Cycas rumphii and Stangeria eriopus, which lack idioblasts in the spo- rophyll parenchyma tissues. The breakdown of idioblasts in the microsporophylls of Macrozamia lucida appears to be exceptional.

Cycad tissues in general are toxic and some in- sect species are known to specialize in feeding on their leaves such as Sierarctia echo, Eumaeus atala florida, and in Mexico, Eumaeus debora, all Lep- idoptera. These insects sequester cycad toxins (TEAS 1967; ROTHSCHILDet al. 1986) but E. debora rarely

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DUNCAN,M. W. , S . P. MARKEY, M. LEVY, I. J . KOPIN, and K. NORSTOG.In press. 2-amino-3-(methylamino)-propanoic acid (BMAA) cycad leaves: quantification in Australian and Mexican cycads responsible for causing hind limb paralysis in cattle. Neurology.

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feeds on cycad cones in Mexico. Though secretory idioblasts may be found in other cycad tissues such as leaves, their concentration is not the same as in the sporophylls. In 2. furjiuracea, the absence of idioblasts in the stem tissue and, at best, few idio- blasts in leaflet tissue is significant. This supports the hypothesis that the high concentration of idio- blasts in sporophyll tissue is probably related to the coevolution of a complex pollination symbiosis in- volving highly destructive insect species such as weevils.

Vascular plants especially angiosperms that are insect pollinated, have developed specialized mechanisms to attract insects to flowers. These mechanisms range from simple nonspecialized, ra- dial symmetrical flowers (Ranunculus type), which are pollinated by a number of different insect spe- cies, to the most complex and highly specialized systems found in orchids, where a given orchid species has coevolved with a given insect species. The rewards the insects receive mostly are in the form of food, pollen, brood place, and shelter (FAEGRIand VANDER PIJL 1979). This seems to hold also for at least some cycads, where the pol- len- and starch-rich staminate cones serve as shel- tering sites and brood places where the insects complete their life cycles (NORSTOGand FAWCETT 1989).

Acknowledgments

I am deeply grateful to KNUTNORSTOGfor his guidance and advice throughout this research and JACKB. FISHERfor carefully revising the manu- script. I thank Fairchild Tropical Garden for lab- oratory facilities and access to the living cycad col- lection. I give special thanks to NELLJENNINGSand ARTHURMONTGOMERYwhose financial support from the Montgomery Foundation made this research possible.

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