178 



EVOLUTION OF PLANTS 



rounded by the floral envelopes, corolla and calyx, 

 made up respectively of the petals and sepals. The 

 base of the carpel forms the ovary, while above it is 

 prolonged into the style tipped 

 by the stigma, or portion upon 

 vsrhich the pollen falls (Fig. 44, 

 A). 



In the number of parts in the 

 flower, as well as in their ar- 

 rangement and form, the Angio- 

 sperms show almost infinite 

 variety. The petals are very 

 frequently brightly colored, and 

 this, together with many modi- 

 fications in the other structures, 

 is intimately associated with 

 pollination through insect aid, 

 which has undoubtedly played 

 an important part in the evolu- 

 tion of the floral structures of 

 the Angiosperms. 



The gametophyte in the An- 

 giosperms is so much reduced 

 and so inconspicuous that it is 

 usually quite ignored in the 

 ordinary study of these plants ; 

 but it must be borne in mind 

 that the gametophyte is always 

 present, although in a verj^ re- 

 duced form. As in the Gymnosperms, the ovule cor- 

 responds to the macrosporangium of the heterosporous 

 Pteridophytes, and within it is formed the single macro- 



FiG. 43. — Flowers of a pond- 

 weed (Naias) ; the male 

 flower, A, and the female, 

 B, are much alike. Each 

 consists of a single sporan- 

 gium invested with an in- 

 tegument, in, the whole 

 enclosed in a tubular leaf 

 with spiny processes at the 

 summit. This sheath-like 

 leaf is usually considered 

 in the female flower to he 

 a carpel. Fig. C shows a 

 section through the base of 

 the female flower, with the 

 enclosed macrosporangi- 

 um, or ovule, ma, and two 

 integuments, in, and the 

 contained embryo-sac, or 

 macrospore, sp. 



