224 - Multicellular Planfs 



The developmental late ol the embryonic 

 parts likewise varies in different seeds. In the 

 pea and coin, for example, the cotyledons 

 remain below the ground as the seedling de- 

 velops, and thus the hypocotyl gives rise only 

 to the root system, and the epicotyl produces 

 all the stem and leal systems (Figs. 12-21 and 

 12-25). But in the bean and pine, the cotyle- 

 dons are lilted above ground, and thus the 

 hypocotyl gives rise to part of the stem, as 

 well as to the root system (Fig. 12-2(1). In the 

 pines and many other plants, the cotyledons 

 develop chlorophyll; and in some cases the 

 cotyledons persist as foliage leaves, alter their 

 reserves of organic matter have been ex- 

 hausted. 



Variations of Floral Structure. Pollination has 

 been a main factor in determining the evo- 

 lution of the floral structure. Some species 

 practice self-pollination, in which case the 



floral envelopes (calyx and corolla) tend to 

 remain closed when the pollen is liberated. 

 Btit a great majority of species depend upon 

 cross-pollination, and the main vectors of 

 pollen from plant to plant are the wind and 

 animals, chiefly insects. 



In general, blight and conspicuous petals 

 (or sepals), distinct fragrances, and nectar 

 glands represent adaptations that have arisen 

 from a long evolutionary association between 

 the angiosperms and insects. As is well 

 known, many insects fly from flower to flower, 

 seeking food (nectar and pollen), and inci- 

 dentally carrying the pollen. In some cases, 

 one finds a very specific relation between 

 the form of the flower and that of the insect 

 vector (Fig. 12-2(i). 



Some plants possess modified leaves near 

 the Mower proper, and sometimes these bracts 

 are bright and showy, usurping the insect- 



SHOOT 



ADVENTITIOUS ROOT 

 SOIL SURFACE 



EPICOTYL 

 GRAIN 



HYPOCOTYL 



SECONDARY ROOTS 

 PRIMARY ROOT 



JfKOUIS KtMvW 



Fig. 12-24. Germination of corn grain. Compare with Fig. 12-23. 



