The Orderly Cycles Pursued in Growth 361 



final direction, by the stimulus of light, to which they set their 

 blades at right angles. As the leaves develop, embryonic tissue 

 in their axils becomes active, and develops into new buds pre- 

 cisely like the first bud produced by the embryo, — the stimulus 

 thereto being probably the pressure ex- 

 erted upon them by the developing leaf. 

 It is easy to see an advantage in this 

 axillary position of buds, for their first 

 need is abundance of food, and the leaves 

 are the source of supply. From these 

 buds grow branches, the primary direc- 

 tions of which are assumed under guid- 

 ance of gravitation, whereby they are 

 sent radiately out into positions of least 

 interference with one another, although 

 the details of their ultimate positions are 

 fixed by a variety of minor influences, 

 precisely as in the case of the root- 

 branches above mentioned. 



Such is the complete structure of a 

 seedling, of which a typical example is 

 here represented (figure 138). 



4. The Growth Cycle: the Adult. — The 

 seedling continues development and 

 growth for a considerable time in the 

 manner just described, branching con- 

 tinuously into new roots and stems, and 

 making new leaves. Its transition to 

 the adult condition may be considered as marked by the beginning 

 of reproduction, even though the plant may by no means have 

 reached its full size. Suddenly, at some time in the plant's growth, 

 without any apparent reason, some buds begin to produce flowers 

 instead of more leaves. The central features of flowers are in 

 reality the pollen grains and the embryo-sacs, and there can be 



Fig. 138. — A typical seedling 

 (of a Maple), showing the 

 distinctive parts, excepting 

 that the axillary buds are 

 not sufficiently visible. (Cop- 

 ied from Gray's Structural 

 Botany.) 



