Reproduction in Multicellular Plants - 231 



EPICOTYL 



COTYLEDON 



SUSPENSOR 



ZYGOTE DEVELOPING 



Fig. 12-34. Development 

 of the embryo of a dicoty- 

 ledonous plant. 



Germination. When a seed is ripe, devel- 

 opment stops temporarily. Under natural 

 conditions most seeds remain in the dormant 

 state throughout the winter or dry season; 

 and some seeds cannot be forced to germinate 

 without the intervention of a definite resting 

 period. At the proper time, with the advent 

 of warmth and moisture, the seed absorbs 

 water, swells powerfully to burst the seed 

 coats, and now the embryo resumes devel- 

 opment. 



The hypocotyl elongates rapidly, forming 

 the primary root, which soon develops a zone 

 of root hairs just above the tip (Fig. 12-20). 

 The primary root is positively geotropic; that 

 is, it grows downward, toward the earth, re- 

 gardless of the position in which the seed 

 may happen to lie (Fig. 12-35 A). At the same 

 time, or shortly later, the epicotyl, being 

 negatively geotropic (Fig. 12-35B), begins to 

 grow upward, forming the primary stem and 

 the leaves. During these early stages of growth 

 the young sporophyte draws upon the re- 

 serve organic nutrients stored in the cotyle- 

 dons and endosperm (if present). This or- 

 ganic material is partly oxidized for energy 

 and partly transformed into essential organic 

 components in the growing cells. Despite the 

 fact that the total amount of organic material 

 is decreasing during this period, the actual 

 size of the young plant increases greatly, 



HYPOCOTYL 



YOUNG EMBRYO 



RADICLE- 

 SUSPENSOR" 



OLDER EMBRYO 



owing to the fact that a large amount of 

 water is absorbed as the new protoplasm is 

 formed. In some plants the cotyledons, after 

 giving up their organic reserves, develop into 

 ordinary foliage leaves, although more often 

 the cotyledons shrivel and finally drop off. 

 In either case, chlorophyll forms in the 

 young leaves (and stem) shortly after they 

 reach the light, and when the organic re- 

 serves originally present in the seed are 



B 



Fig. 12-35. A, positive geotropism in primary root of 

 the bean. B, negative geotropism in young stem of 

 bean plant. 



