INTERSPERSED SEXUALITY IN RICINUSlJ

WILLIAM L. GEORGE, JR.3 AND OVED SHIFRISS

Department of Horticulture and Forestry, Rutgers-The State University, New Brunswick, New Jersey 08903

Received May 31, 1967

T H E castor-oil plant, Ricinus communis L., is potentially ever-flowering. The inflorescences of standard varieties exhibit gradient monoecism, i.e., the basal

branches of each inflorescence differentiate staminate flowers exclusively and the apical branches, pistillate flowers. Another form of monoecism is interspersed. In this form all branches of the inflorescence differentiate pistillate flowers and, in addition, each branch is capable of differentiating some staminate flowers.

It is difficult to analyze the inheritance of the factors governing interspersed sexuality because the differentiation of interspersed staminate flowers is quanti- tative in nature and is greatly affected by nongenetic variations. SHIFRISS (1956, 1960) concluded that interspersed sexuality results from a combination of heredi- tary factors for femaleness and genes for interspersed staminate flowers (id). He suggested that some id genes are environmentally sensitive and others are en- vironmentally resistant. But these genes have not been identified. ZIMMERMAN and SMITH (1966) postulated that the differentiation of environmentally sensi- tive interspersed staminate flowers is determined by a system of polygenes.

The present paper reports on the finding of two independent genes, id, and id,, whose combined action in some genetic backgrounds results in a high level of expressivity of interspersed staminate flowers under a wide range of environ- mental conditions. The behavior and possible usefulness of these genes will be discussed.

M A T E R I A L S A N D M E T H O D S

Termindogy: The castorbean inflorescence is a raceme of cymes. There are four kinds of racemes with regard to mode of sex differentiation and these are designated by symbols A, B, C, and D (Figure 1). Racemes A and C exhibit apical distribution of pistillate flowers and racemes B and D exhibit uniform distribution. The non-interspersed racemes are A and B; the inter- spersed racemes are C and D; C being apically interspersed and D uniformly interspersed. Al- though both C and D are classified here as interspersed phenotypes, only D represents the com- nimly kmwn form of interspersed sexuality.

There is considerable variation in level of expressivity of interspersed staminate flowers. Level of expressivity is measured by the percentage of interspersed staminate flowers.

Apical distribution of pistillate flowers is associated with gene M O for gradient monoecism.

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348

Uniform distribution of pistillate flowers is associated with either mo/mo (SHIFRISS 1960) or (MO). The symbol (MO) is tentative. I t stands for a nuclear factor of some kind which reduces the expression of MO (SHIFRISS 1966).

W. L. GEORGE AND 0. SHIFRISS

- - - . - ~

FIGURE 1.-Racemes of castorbeans illustrating four patterns of sex differentiation. In the diagrams (hottom row) each open circle represents a cluster of staminate flowers and each solid square. a cluster of pistillate flowers. A, apical distrihution of pistillate flowers (gradient monoe- cism); B. strictly pistillate; C, interspersed staminate flowers in the apical pistillate region; and D, entirely interspersed (after SHIFRISS 1966).

SEXUALITY I N R I C I N U S 349

Material: The interspersed parent was Si-2. This is a single perennial tree bearing D racemes. It was selected by one of us (O.S.) in the seventh inbred generation of the cultivar US 3/415-9. This cultivar is completely stable with regard to uniform distribution of pistillate flowers. The genetic control of uniform distribution in this case is not yet understood, but there is little doubt that US. 3/415-9 carries the factor which reduces gene expression at locus MO. The seed was obtained from DR. LEROY H. ZIMMERMAN of the U.S. Department of Agriculture.

Three non-interspersed parents were employed: (1) ES3, a standard monoecious inbred of the variety Early Spineless (Syn. Mautner) which differentiates A racemes, Mo/Mo. (2) NF1-6, a female tree mo/mo of N145-4 which differentiates B racemes. Variety N145-4 is a backcross population, mo/mo x Mo/mo. It was described in this country by CLAASSEN and HOFFMAN (1950) but it is probably identical to the material used by KATAYAMA (1948). Symbol mo is synonymous to a of KATAYAMA and f of SHIFRISS (1956). (3) GSR5, a genetically unstable inbred of the cultivar Gamadon. This inbred consists of A and B plants as well as sex-reversals which change from B to A racemes at different times during plant development (SHIFRISS 1960). It is assumed that the phenotypic change from B to A in these sex-reversals is associated with a nuclear change from ( M O ) to MO.

Consfant enuironment: Some of the experimental material was grown under controlled con- ditions of continuous illumination, constant temperature, and nutrient regime ( SHIFRISS 1964.). This specific environment will be known hereafter as the constant environment.

Selection experiment: Preliminary observations have shown that the inbred offspring of tree S,-2 varies greatly in expressivity of interspersed staminate flowers. Therefore, an attempt was made to select from this material two extremely different interspersed inbreds of high and low levels of expressivity.

The inbred offspring of tree S,-2 varies also in node number. The term node number refers to the number of n7d.s on the main stem from the mtyledon leaves to the primary raceme. This is a measure of time of flowering. Plants which flower at different node numbers may be affected by different nongenetic variations during sex differentiation. We therefore attempted to select high and low interspersed inbreds which would flower at the same node number under comparable environmental conditions.

Znheritance: The inheritance studies are largely confined to factors which determine the differentiation of interspersed staminate flowers. The genetics of mode of distribution of pistillate flowers is discussed elsewhere (SHIFRISS 1966). The parents, F,, F,, and backcross populations were grown and classified in the field during the summers of 1964 and 1965. Unless otherwise specified the classification of each plant was based on the phenotype of the first five racemes.

R E S U L T S A N D DISCUSSION

Selection experiment: Forty-eight inbred offspring of tree S,-2 were grown in the constant environment. In this environment the off spring varied in expressivity of interspersed staminate flowers and in node number (9 to 31). Plant Sr-19 exhibited the highest level of expressivity and the lowest node number. The progeny of S,-61, derived from Ss-1 9, proved to be uniform both in node number and in its high level of expressivity of interspersed staminate flowers (Table 1). A more advanced selection, S,,,-3, a derivative of S,-61, has approximately the same level of expressivity and node number (Table 1 ) .

A low interspersed selection was not found in the above experiment. Therefore, a larger inbred progeny of tree S7-2 was grown again in the constant environ- ment and in this s, population a developmentally persistent low interspersed plant, Ss-208, was found. A comparison between the new and high and low interspersed inbreds is presented in Table 2. The high interspersed inbred S,,-lO

350 W. L. GEORGE A N D 0. SHIFRISS

TABLE 1

Selection from US 31415-9 of an interspersed inbred which exhibits a high level of expressiuity of interspersed staminate flowers

Offspring*

Expressivity level Number Node (% interspersed

Year Parent of plants number staminate flowers)

1963 s,-2 sa-19

1964 si-2 s,-19

1965 si-2

SlO-3

S,-61

S,-61

100 100 50

100 100 100 100 100

10.9 & 3.9 7.7 t 0.7

12.4 t 1.8 10.2 k 1.5 9.6 k 1.3 9.2 t 2.4 7.1 t 0.8 6.9 t 0.3

38.9 * 14.7 54.4 f 12.9 4Q.5 f 17.2 67.5 -C 8.8 71.5 f 8.4 55.9 f 14.6 76.0 1. 6.2 76.4 & 4.1

* Plants number of

were grown outdoors in New flowers per raceme was 300.

Brunswick, N.J. Classification was based on the primary racemes. The average

was derived from S1,-3 (Table 1) and the low interspersed inbred S9-80 was derived from Sa-208.

Inheritance: The inheritance of the factors governing the differentiation of interspersed staminate flowers was studied in the following crosses.

Non-interspersed parent Interspersed parent Cross N o . ES3 X s,-2 1 GSRS X s,-2 2 GSRS X S,-61 3 NFI-6 X S,-61 4

F, plants of cross 1 and cross 3 were quantitatively indexed for levels of expres- sivity of interspersed staminate flowers. Ten plants of each of the parents and F, hybrids were grown in the greenhouse under 16 hr daylength. The data for F, of cross 1 were based on 30 racemes per plant and for F, of cross 3 on the primary raceme only. The means for the expressivity (percent interspersed staminate flowers) of the parents and the F,s of cross 1 are: ES3, 0.0; Si-2, 45.1 5 8.4; ES3 x Si-2, 8.9 * 3.4; and Si-2 x ES3, 11.7 * 7.6. The comparable data for cross 3 are: GSRS 0.0; S,-61, 59.6 1. 6.1; and GSRS X Ss-61, 18.0 * 5.1. There

TABLE 2

Ezpressivity of interspersed staminate flowers in high and low inbreds grown in the ccnsfant enuironment

Offspring-

Expresswity level Expressivity Number Node (% interspersed

plants numbers staminate flowers) Parent level of mrent ~

s,,-10 High 10 8.1 +: 0.3 58.8 -t- 5.4 S,-80 Low 10 8.4 f 0.5 3.7 * 2.5

~

* Classification was based on the primary racemes. The average number of flowers per raceme was 335

SEXUALITY IN RICINUS

TABL,E 3

ES3, non-interspersed x S,2, interspersed

35 1

F2 BC,, F,XES3

Individual F, Inter- Non-inter- Inter- Non-inter- plants employed spersed spersed Chi-square spersed spersed Chi-square

(3:l) 22-6t 83 35 1.4 ns 33-3% 91 33 0.2 ns 33-4 97 31 0.0 ns 33-5 107 28 0.8 ns 33-10 80 50 12.6 * *

45 8 177 2.8 ns Heterogeneity x? = 11.6 *

- -

(1 : l ) 63 62 0.0 ns 88 52 9.2 * * 78 63 1.6 ns 55 82 5.3 * 59 83 4.1 *

343 342 0.0 ns Heterogeneity x’ = 20.2 * *

~ _. __

(15:l) (3:l) 22-1 116 11 1.3 ns 116 26 3.4 ns 22-2 86 8 0.8 ns 1 02 35 0.0 ns 22-3 81 2 2.1 n s 121 17 11.8 * * 2.2-4 121 11 1.0 ns 42 19 1.2 ns 33-2 127 9 0.0 ns 91 39 1.7 ns 33-6 78 8 1.4 ns 96 4 6 4.4 * 33-7 128 9 0.0 ns 120 21 7.7 * *

737 58 1.5 ns 688 203 2.3 ns - - - -

Heterogeneity x* = 4.5 ns Heterogeneity x2 = 18.9 * *

11s. nonsignificant: * significant at 57; probability level; * * significant at 1 % probability level. f 22 for F,. S,-2 X ES3. $ 3 3 for F,. ES3X S,-2.

is no significant difference in the level of expressivity of interspersed staminate flowers between reciprocal hybrids of cross 1. Reciprocal hybrids in cross 3 were not possible before sex reversion of parent GSRS.

The F, and backcross data of cross 1 and cross 2 are presented in Tables 3 and 4. The results suggest that tree Si-2, the interspersed parent employed in these two crosses. carries id genes at two independent loci and that one of these genes is in a heterozygous state. We assume that the statistically significant deviations from the expected ratios in some F, and BC, families of cross 1 (Table 3 ) are due to errors in classification. In this cross of apical (A) x uniform (D) the F, is un-

TABLE 4

GSRS, non-interspersed x S,-2, interspersed

F, Chi-square Individual

F, plants employed Interspersed Non-interspersed (3 : l ) (15:i)

509-30 509-142

458 157 929 71

0.1 ns 1.2 ns

ns, nonsignificant.

352 W. L. GEORGE A N D 0. SHIFRISS

stable genetically and phenotypically, changing from uniform to apical racemes at different times during plant development. F, progenies obtained from different racemes of a single Fl consist of varying proportions of apical plants, ranging from 0 to 75% (SHIFRISS 1966). We have found that, within a given family, it is more difficult to ascertain the presence of id genes in apical plants than in uniform plants. Therefore, the classification of progenies which consist of rela- tively high proportions of apical plants is apt to be inaccurate. We shall discuss elsewhere the ontogenetic and environmental factors which contribute to this error in classification.

The F, and BC, progenies of crosses 2, 3, and 4 consisted largely of plants bearing uniform racemes B and D. In these progenies no difficulty has been experienced in classification of the two alternative phenotypes. The interspersed parent employed in crosses 3 and 4 was S,-61. Unlike S,-2, this parent breeds true for a high level of expressivity of interspersed staminate flowers. The inherit- ance data for cross 3 were obtained during 1965. The sex phenotypes in the F, were easily classified into three groups based on comparison with the parents and the F,: non-interspersed, intermediate, and high level of expressivity. The results (Table 5 ) support the hypothesis that the differentiation of interspersed staminate flowers in this cross is determined by two independent pairs of genes designated here as id, and id,. The three clearly distinguished phenotypic groups observed in the F, are assumed to be associated with the following genotypes: non-inter- spersed, Id,/Id,, Ida/Ida; high interspersed expressivity, idl / idl , id2/idg; and intermediate grades of expressivity, all combinations between Id and id genes.

The F, and BC, data of cross 4 also support the duplicate gene hypothesis. In this cross the F, is almost strictly female, differentiating only a few interspersed staminate flowers late in the ontogeny of some racemes. The F, from a single Fl tree consisted of 141 plants with uniform racemes: 127 individuals being inter- spersed of different grades and 14 strictly non-interspersed. Five F, trees were backcrossed to $61. These backcross progenies consisted of 732 interspersed plants: 190 appeared as the F,, in being almost strictly female, and 542 exhibited graded but higher levels of expressivity.

TABLE 5

GSR5, non-interspersed x S9-61, interspersed

FZ BC,, GSR5 X Fl

Individual High Intermediate Non- Intermediate Non- F. plants employed interspersed interspersed interspersed interspersed interspersed

55710 15 266 14 88 28 55-1 1 7 1 22 8 86 35 55-1 3 13 202 14 108 29 55-14 16 221 14 74 29 55-16 14 220 12 108 34

65 1031 62 1F6.F 155 - - - - -

None of the deviations from the expected 15:l (Fz) and 3:l (BC,) ratios are statistically significant.

SEXUALITY IN RICINUS 353

Female-determining factors and id genes: The F, progenies of cross 1 (A x D) consisted of varying proportions of strictly females (B) as well as apically-inter- spersed plants (C) in addition to A and D plants. Therefore, it is unlikely that one of the id genes or their combination determines alone the differentiation of interspersed sexuality (D) . Rather, the evidence suggests that interspersed sexu- ality is manifested in the presence of factors for uniform distribution of pistillate flowers, the female-determining factors, and id genes for interspersed staminate flowers.

Expressiuity levels: The F, of crosses 3 and 4 consisted entirely of B and D plants. However, the expressivity level of interspersed staminate flowers was con- siderably higher in the F, of cross 3 than in the F, of cross 4 grown under the same field conditions. Cross 3 is GSR5 xSg-61 and cross 4 is NF1-6 x Sg-61. The lower expressivity level of interspersed staminate flowers in the F, of cross 4 could be due to factors residing in the genetic background of female NF1-6.

In order to ascertain the possible relationship between level of expressivity on the one hand and potency and dosage of id genes on the other, the intermediate group in the F, of cross 3 (Table 5) was reclassified in the field into three graded phenotypes. This finer classification was based on careful comparison between each F, plant with the F, and the parents. The parents, F,, F,, and BC, were all grown under the same field conditions. On the basis of this new classification we obtained five phenotypes in the F, and three phenotypes in the BC, (Table 6) . The five phenotypes were distinguished as follows: B, strictly non-interspersed as parent GSR5; BD, extremely low level of expressivity, intermediate between GSR5 and the F,; DL, low level as in the F,; DM, medium level, intermediate between F, and Sg-61; and DH, high level as in parent S,-61.

The results (Table 6) suggest a ratio of 1:4:6:4:1 for each F, family and a ratio of 1:2:1 for each BC, family. However, whereas the heterogeneity x2 for indi- vidual F, families is not statistically significant, the x2 for the total is highly sig- nificant. A closer examination of the differences between the numbers of the

TABLE 6

GSRS, non-interspersed x S,-bI, interspersed. Reclassification of data presented in Table 5

F? BC,, GSR5 X F,

Number in classes+ Number in classes) Individual Chi-square Chi-square

F, plants employed DH DM DL BD B (1:4:6:4:1) DL BD B (1:Z:l)

55-10 15 68 116 82 14 3.3 ns 55-1 1 7 24 57 41 8 5.3 ns 55-13 13 4.9 102 51 14 5.0 ns 55-14 16 54 110 57 14 4.6 ns 55-16 14 60 109 51 12 5.7 ns

65 225 494 282 62 14.8** Heterogeneity x* = 24.1 ns

- - _ - -

27 61 28 0.3 ns 1.7 ns 25 61 35

36 72 29 1.1 ns 21 53 29 1.3 ns 34 74 34. 0.3 ns

143 321 155 1.4 ns - - -. -

Heterogeneity x 2 = 4.5 ns

ns, nonsignificant; * * significant at 1 % probability level t See text for description of phenotypic classes.

354 W. L. GEORGE A N D 0. SHIFFUSS

observed and expected phenotypes shows a deficiency of plants in the DM class of each F, family, This deficiency amounts to a total of 35 plants. The DL class has an excess of 60 plants in total.

Alternative hypotheses are presented in Table 7 to account for the results in Table 6. Both hypotheses are based on the assumption that the expressivity level increases with dosage of id genes. The hypotheses differ with regard to potency of these genes. In hypothesis 1 we assume that id, is equal in potency to id,. In hypothesis 2 we assume differential potency and assign expressivity levels of 31 % to id, and 19% to id,, i.e., a differential of +6%. We chose a differential potency of 6% because it accounts for errors in classification without greatly affecting the observed F, ratio of 1 :4:6:4: 1 (Table 6).

Based on differential potency (Table 7, hypothesis 2) one would expect some difficulty in distinguishing between certain genotypes of groups BD and DL. A more serious difficulty would be expected in the classification of the DM group of plants. The genotypes of 69% expressivity constitute one half of the D" group and these would be extremely difficult to distinguish from the majority of the DL group.

The assignment of quantitative values to the id genes is somewhat artificial as it oversimplifies the nature of gene action. Nevertheless, it seems to us that the hypothesis of differential potency better accounts for the continuous variation observed in the F,. It also helps us visualize how a quantitatively manifested character is qualitatively inherited.

From cross 2 we extracted a new D line which breeds true for an intermediate

TABLE 7

Interpretation of the F, data presented in Table 6 IdJId , , Id,/Id, X id,/id,, id,/id,

Levels of expressivity

Hypothesis 1 Hypothesis 2 Phenotypic class Genomtype

B Id,/Id, , Id,/Id, 0 0

BD Id,/Id, , Id,/id, 25 19 Id,/Id, , Id,/id, 25 19 Id,/ id, , Id,/Id, 25 31 Id,/ id, , Id,/ld, 25 31

Id,/ id, , Id,/id, 50 50 DL Id,/Id, , id,/id, 50 38

Id , / id , , Id,/id, 50 50 Id,/ id, , Id,/id, 50 50 Id,/ id, , Id,/id, 50 50 id,/id, , Id,/Id, 50 62

D" Id,/ id, , id,/id, 75 69 Id,/ id, , id,/id, 75 69 id, / id, , Id,/id, 75 81 id, / id, , Id,/id, 75 81

DH id,/id, , id,/id, 100 100

SEXUALITY IN RICINUS 355

level of expressivity of interspersed staminate flowers between the low and high inbreds of US 3/415-9 (Table 2) but closer to the high inbred. We might there- fore be able to test the hypothesis of differential potency by crossing the low and intermediate interspersed inbreds.

Genetic instability and id genes: Genetic instability affecting the expression of gene MO is a common phenomenon in Ricinus. Genetically unstable sex-reversals were found in the F,, F,, and backcross populations of crosses 1, 2, and 3. These sex-reversals change during plant development from B or D to A or C racemes. The results in Table 8 suggest that this instability does not affect the inheritance of the id genes.

The id genes in Ricinus: Several observations suggest that in addition to id, and id, there exist in castorbeans other genes for interspersed staminate flowers. SHIFRISS (1960) developed inbreds which are female in one environment and highly interspersed in another. These inbreds were derived from ( M o ) / ( M o ) mutants found in North-African and Israeli populations. Phenotypically similar inbreds were developed by ZIMMERMAN and SMITH (1966) but their lines are mo/mo. In both cases the genetic mechanism governing the differentiation of environmentally-sensitive interspersed staminate flowers is not well understood. Comparable tests in New Jersey and Puerto Rico proved that the North-African and Israeli inbreds are considerably more sensitive to environmental conditions than the low and high interspersed inbreds of US 3/415-9.

Spontaneous female mutants of the (Mo) / (Mo) type occur commonly in wild populations and inbred lines of Ricinus. It is conceivable that genes for inter- spersed staminate flowers have a selective advantage under natural conditions, because, in the presence of these genes, such mutants can reproduce their kind. Agriculturally, the combination between factors for femaleness and id genes is of considerable value. First, interspersed inbreds such as S,-61, icll/idl, id2/id2, are superior in yield to comparable inbreds which exhibit gradient monoecism. Second, environmentally sensitive id genes can be used successfully in hybrid seed production.

TABLE 8

Inheritance of interspersed staminate flowers in relation to phenotypic reversion Non-interspersed x interspersed

F, seu-reversals Phenotype of F, of c ros racemes employed

1 (plant 22-4) D. before reversion

C, after reversion 2

(plant 509-142) D, before reversion C, after reversion

-

Inter- spersed

61 1598

195 133

Fz BC,. F, X non-interspersed parent

Non- Non- inter- Chi-square Inter- Inter- Chi-square

spersrd (15.1) spersed spersed (3.1)

4 0.0 87 25 0.4 12 0.2 84 30 0.6

10 0.7 142 4.0 0.9 12 1.5

None of the chi-square ralnes is significant

356 W. L. GEORGE A N D 0. SHIFRISS

Interspersed sexuality in other plants: Some monoecious forms in Amaranthus (MURRAY 1940), Spinacia, and perhaps other plant genera (see reviews by ALLEN 1940 and WESTERGAARD 1958) are similar to the interspersed inbreds in Ricinus. The standard sex form in Spinacia obracea is dioecious but some varieties consist of a graded series of monoecious plants which appear as modified females. The differentiation of staminate flowers in these plants is greatly affected by environmental fluctuations. IIZUKA and JANICK ( 1962) presented evidence which suggests that the differentiation of femaleness, monoecism, and maleness is deter- mined by three allelic genes. However, the genetics of some monoecious variants in this species is not yet well understood.

S U M M A R Y

The interspersed pattern of sex differentiation in the inflorescences of this plant is determined by hereditary factors for uniform distribution of pistillate flowers and genes for interspersed staminate flowers.-In crosses between a per- sistently high interspersed inbred derived from US 3/415-9 and some strictly non- interspersed inbreds, the alternative characteristics are governed by genes at two independent loci; Idl/Idl, Id,/Id2 being associated with the non-interspersed and idl/idl, id,/id, with the high interspersed phenotype.-The level of expressivity of interspersed staminate flowers appears to depend on the dosage of the id genes, their locus, and the environment.-In addition to id, and ids there exist other genes for interspersed staminate flowers whose expression is particularly sensitive to environmental fluctuations.-The id genes may be advantageous for the organ- ism when in combination with factors for uniform distribution of pistillate flowers.

L I T E R A T U R E CITED

ALLEN, C. E., 1940 The genotypic basis of sex-expression in angiosperms. Botan. Rev. 6:

CLAASSEN, C. E., and A. HOFFMAN, 1950 The inheritance of the pistillate character in castors and its possible utilization in the production of commercial hybrid seed. Agron. J. 42 : 79-82.

IIZUKA, M., and J. JANICK, 1962 Cytogenetic analysis of $ex determination in Spinacia oleracea. Genetics 47: 1225-1241.

KATAYAMA, Y., 1948 The progeny of female individuals in castorbeans. Japan. J. Genet. 23: 19 (in Japanese).

MURRAY, M. J., 1944l The genetics of sex determination in the family Amaranthaceae. Genetics 25: 409-431.

SHIFRISS, O., 1956 Sex instability in Ricinus. Genetics 41: 265-280. - 1960 Conven- tional and unconventional systems controlling sex variations in Ricinus. J. Genet. 57 : 361-388. __ 1964 Growth and sexuality of Ricinus communis L. in a constant en- vironment. Am. Naturalist 98: 187-189. __ 1966 Synthesis of a new system of sex- reversals in Ricinus communis L. Mimeographed manuscript in the Department of Horti- culture and Forestry, Rutgers-The State University, New Brunswick, N.J.

The mechanism of sex determinatioE in dioecious flowering plants. Advan. Genet. 8: 217-281.

Production of F, seed in castor-beans by use of sex genes sensitive to environment. Crop Sci. 6: 4 0 6 4 9 .

227-300.

WESTERGAARD, M., 1958

ZIMMERMAN, L. H., and J. D. SMITH, 1966