Differentiation 185 



The difference between these two types of structure begins at the 

 meristem and may often be recognized there by the presence of a large 

 number of axillary buds which are destined to be flower buds. The de- 

 velopment of the floral apex has been studied by many workers (p. 67). 

 A single flower is a modified axis, and its parts arise from primordia 

 which, although limited in number, are distributed in a precise pattern. 

 The differentiation between them takes place early and produces sepals, 

 petals, stamens, and carpels. The origin of these parts from particular 

 layers of the meristem has been worked out, through the aid of chimeras, 

 by Satina and Blakeslee ( p. 272 ) . In abnormal growth some floral organs 

 may be so modified that they resemble others, as in the conversion, partial 

 or complete, of petals to stamens or sepals to leaves (p. 277). 



In many trees the differentiation of flower buds begins very early, 

 usually in the season before the flowers are borne. This is an important 

 matter for horticulturalists and has been extensively studied (Zeller, 

 1954, and others), since environmental conditions favoring flower pro- 

 duction must be provided early. Whether a tree will flower (and 

 fruit) well in a given season is often determined in June of the year 

 before. 



Implicit in the process of reproduction is the differentiation between 

 the sexes. In plants with perfect flowers this is evident only in the dif- 

 ference between stamens and pistils. In monoecious plants, there are two 

 kinds of flowers on the same plant and in dioecious ones these are on dif- 

 ferent plants. The significance of such differences lies in the various 

 mechanisms that tend to accomplish pollination, in many cases cross- 

 pollination. A genetic basis has been found for some of these and is 

 doubtless present in others. Environmental factors of various sorts are 

 also operative here, notably nutrition and light. It is sometimes possible, 

 for example, to change a staminate into a pistillate plant by altering the 

 photoperiod (p. 317). The existence of sexual reproduction itself, in con- 

 trast with the much less precarious method of vegetative reproduction, is 

 based on the presumptive advantage of the higher variability that results 

 from the recombination of genetic potencies following svngamy and 

 meiosis. 



Other traits are sometimes associated with the fundamental difference 

 between the sexes, as observed in Mercurialis annua (Basarman, 1946), 

 Valeriana dioica (Moewus, 1947), Urtica dioica, and Rumex acetosa 

 (Umrath, 1953; Fig. 8-1). In general, the female plants are larger and 

 are also different from the males in the size and shape of their leaves. 



A conspicuous example of external differentiation, since it involves an 

 entire plant body, is that between the gametophyte and the sporophyte. 

 In many of the simpler plants the two generations are very much alike, 

 but they are markedly different in bryophytes and vascular plants. The 



