428 Morpho genetic Factors 



species at the base and thus like the one to which the nucleus belongs. 

 Evidently the nucleus, with its genes, determines the form of the hat 

 (Fig. 19-6). The delay in expressing this determination is thought to be 

 due to the persistence for a time of specific substances in the cytoplasm 

 of the stalk, produced previously by the nucleus of the species from which 

 it came. Acetabularia has provided material for many experiments im- 

 portant for genetics and physiology as well as for morphogenesis, most 

 of them by Hammerling ( see, for example, 1946 ) . 



Genes and Sex Structures. A particularly complex problem in genetics 

 and one of much importance in morphogenesis is the inheritance of sex 

 and the determination of the structures in which sexual differences are 

 expressed. 



In animals most individuals definitely belong to one sex or the other, 

 and the production of both male and female gametes by the same or- 

 ganism is rare. In plants, however, it is much commoner than the uni- 

 sexual condition. Among higher forms, staminate and pistillate flowers 

 may be separate but on the same individual (the monoecious condition) 

 or the flowers may be perfect and hermaphroditic. There are a consider- 

 able number of cases, however, where the two types of sexual structures 

 are borne on different individuals (the dioecious condition). Such forms 

 are strictly comparable to unisexual animals. 



In determining just what the sexual character of a plant will be, how- 

 ever, the environment has a much greater effect than it does in animals. 

 Sex reversal or the production of one sort of sexual structures rather 

 than another due to physiological or environmental changes is rare in ani- 

 mals but in plants this is relatively easy to accomplish. In monoecious 

 angiosperms the ratio of staminate and pistillate flowers to each other 

 or to the perfect flowers which sometimes occur on such plants may be 

 determined by light or by growth substances, or it may be an aspect 

 of the general phasic development of the plant (as in cucurbit flowers; 

 Nitsch, Kurtz, Liverman, and Went, 1952 ) . The problem of sex expression 

 in plants is therefore in large measure a developmental rather than a 

 genetic one. Although there is a definite genetic basis for most of the 

 sexual differences, this wide variability in its expression makes genetic 

 analysis particularly difficult. There is a large literature on this subject 

 only a very small part of which can be mentioned here. 



With the rediscovery of Mendel's law there were many attempts to 

 analyze sexual differences in dioecious plants in mendelian terms. Among 

 the early workers were Correns (1907), Bateson (1909), and Shull 

 (1910). The plants on which they first worked were Melandrium (Lych- 

 nis) and Bryonia. Correns concluded from his experiments that in Melan- 

 drium the egg cells all carry a tendency toward femaleness and that the 

 male is heterozygous for sex, half the male gametes bearing female 



