THE DIFFERENTIATION OF FLOWER-BUDS 35 



calyx-cup as it is sometimes called, and the carpel or carpels, 

 are not so intimately united during development as they are 

 in the pomaceous types. While there is considei-able diver- 

 sity in morphology among the various wild species, the 

 cultivated varieties are readily placed in one group or the 

 other. The apple, therefore, may well serve as a basis for 

 following out the sequence of developmental changes which 

 take place in the differentiation of the fruit-bud and floral 

 parts in the more common deciduous tree-fruits. 



36. The apple. — Just prior to the differentiation of the 

 parts of the flower-bud, it is not possible to distinguish 

 between those growing points from 

 which flower primordia will be de- 

 veloped and those which will remain 

 as vegetative growing points. Each 

 shows a smooth rounded crown of 

 meristematic tissue more or less in- 

 closed by the beginnings of leaves or 

 bud-scales. As the season progresses, 



the axis from which a flower-bud 



.,, , , 1 11 1 r Fig. 13. — Floral diagram 



will develop gradually becomes dis- f fh • i 



tinguishable through what appears 



to be a thickening or more broadly rounding or flattening- 



out of the growing apex or the crown (growing point), and 



soon thereafter the contour of this crown becomes slightly 



irregular or papillatcd, due to several new growing points 



becoming organized, which now proceed to develop into new 



axes (the individual flower primordia) and on these in turn 



growing points give rise to the individual floral parts and 



tissues. The development of the floral parts is acropetal, 



wliich means that they are differentiated in the same sequence 



as they occur in the fully developed flower. (See Fig. 13.) 



Thus their order of development is as follows: calyx (sepals), 



corolla (petals), stamens and pistils (carpels). As the pistils 



