[Chap. XXXI INITIATION OF FLOWERS 335 



In tliis chapter the initiation of flower primordia will be given most 

 attention. The further development of these primordia to full-grown 

 flower structures and fruits will be mentioned occasionally. 



We shall have to turn our attention once more to the growing stem 

 tip, for it is from this meristem that all flower primordia develop. They 

 may develop from the stem tip in the terminal buds of main stems or of 

 lateral branches, as in roses, zinnias, dahlias, petunias, and asters. In 

 these plants there is a change from a vegetative bud to a flower bud. 



Flower primordia may also develop from the unelongated stem tips of 

 axillary buds, as in rose of Sharon, hibiscus, hollyhock, bindweed, mul- 

 lein, and coleus. In these latter plants the fully developed flowers are 

 later seen along the sides of the stem in the axils of leaves. The leaves 

 subtending the axillary flowers may be large, as in hibiscus, or they may 

 be small green bracts, as in coleus. The stem tips from which these flowers 

 developed never bore leaves. 



Observable differentiation of flower primordia. In the longitudinal sec- 

 tion of a vegetative bud (Fig. 32) the apical meristem and the primordia 

 of the lateral foliage leaves and of axillary buds are evident. When such 

 a bud changes to a flower bud, no more primordia of foliage leaves and 

 axillary buds are formed; but a number of small mounds or ridges of 

 cells develop from the apical meristem in regular arrangement. In the 

 simplest cases the lowest and outermost mounds of these cells are the 

 primordia of the sepals of the flower, and they are usually the first to 

 appear (Fig. 140). Just above them are the primordia of the petals of 

 the flower, then the primordia of stamens; and finally the center of the 

 meristem becomes the primordium of the pistil. The order in which 

 these different primordia become visible is not the same in all species of 

 plants. This short stem tip, with the several mounds of meristematic 

 cells, is the beginning of a flower. 



Physiological differentiation precedes the occurrence of flower pri- 

 mordia. Back of all visible changes in development there are, of course, 

 changes in phvsiological processes and conditions. What we see are 

 merely the consequences of the accumulated physiological conditions. 

 What are these conditions? 



Among the conditions postulated to be important in flower formation, 

 two — namely, specific hormones called "florigens," and the relative 

 amounts of carbohydrates and proteins in the plants — have been given 

 the most attention in recent years. While an unqualified specific answer 



