510 The Determination of Sex 



though the expression of one or the other tendency seems to 

 prevail in certain cells and regions. Here sex is largely a de- 

 velopmental problem. As a perfect flower develops, certain cells 

 become anthers and contain microspores whereas others become 

 carpels and contain megaspores. Which type of spore is pro- 

 duced in a given region is apparently determined chiefly by the 

 position of that region relative to the remainder of the flower. 

 This pattern of development of the various parts of the flower 

 is under the control of genes which are not to be considered as 

 male- or female-determining. In monoecious plants with im- 

 perfect flowers such as maize the position of the region relative 

 to the rest of the plant is apparently the chief determining fac- 

 tor, and it, too, is a matter of the ontogeny of the plant. Thus, 

 as the terminal inflorescence is initiated, microspores become 

 developed in it, whereas the lateral inflorescences generally con- 

 tain only megaspores. In other words, in hermaphroditic plants 

 the presence of a particular type of sex organ in a certain region 

 of the plant is largely a problem of development in an organism 

 in which both potentialities are present and in which sex is 

 largely determined by the ontogenetic pattern. Even as a par- 

 ticular region develops, occasional deviations in the usual onto- 

 genetic pattern may result in the development of organs of the 

 opposite sex from that which normally is produced at a given 

 point. Ears of maize are frequently terminated by a male spike, 

 and female flowers and occasionally small ears may be found in 

 tassels. Figure 146 illustrates a perfect small ear which has 

 been produced in a tassel that also contains a number of male 

 flowers. This ear illustrates the possibilities of the development 

 of organs of the opposite sex in a definite region of a monoecious 

 plant. Seeds are often produced in a maize tassel when the 

 plants are grown during a shortened day. 



Some interesting changes in the sexual condition of certain 

 plants have been brought about experimentally by changes in 

 the environment or by the manipulation of the genotype. Hemp 

 is a dioecious plant whose sex expression has been studied rather 

 intensively. In a lung series of experiments Schaffner showed 

 that length of day and other environmental factors could modify 

 the sexual expression to the extent of producing some flowers of 

 one sex on plants of the opposite sex. Similar modifications of 

 sex expression in hemp by changes in length of day were obtained 



