Ch. VIII, 6] CYCLE OF DEVELOPMENT 389 



branches with one another, partly because of accidents, and 

 partly because of phototropic and other self-adjustments. 

 The real mark of adult age is the beginning of sexual repro- 

 duction. After the young plant has attained a considerable 

 growth, presumably accumulating food in reserve, some 

 of the axillary buds, precisely ahke in position and mode of 

 formation to those which have been producing leafy branches, 

 begin to produce flowers, — that is, specialized determinate 

 branches containing reproductive spores which develop the 

 sexual cells. As to the nature of the stimulus which leads 

 the plant thus suddenly to convert certain of its branch 

 buds into flower buds, or more exactly, to develop reproduc- 

 tive spores with the correlated floral structures, we have as 

 yet no exact knowledge, although the influence of various 

 external factors is clearly apparent. Having once begun to 

 produce the flowers, the plant continues to make them, just 

 as it makes leaves, branches, and roots, as long as it lives. 

 The central parts of these flowers are pollen grains and em- 

 bryo sacs, which in turn develop the two kinds of sex cells. 



The next stage in the cj^cle includes fertilization. The 

 floral parts are essentially organs functionally fitted to effect 

 union of the sex cells, — and a union usually between two 

 different parental strains. By utilization of the motive 

 power of winds, insects, etc., the pollen containing the sperm 

 cell is transported from its place of formation to the vicinity 

 of the deeply-buried egg cell, after which the growth of a 

 pollen tube brings egg cell and sperm cell together into a 



single FERTILIZED EGG CELL. 



The next stage is that of the development of the fertilized 

 egg cell into an embryo. The s'ngle cell, Ijang in the 

 embryo sac, begins at once to divide and to grow, then 

 divides again and grows farther, and thus, under guidance 

 of influences partly hereditary and partly environmental, 

 it gradually assumes the form of the many-celled embryo, 

 with its stem and cotyledons. Meantime the endosperm or 

 food substance is forming around the embryo, and the hard 



