Plant World Continued 



243 



The fertilized egg, now called a zygote, gives rise to the embryo 

 which consists of a cylindrical ste*m with narrow whorled leaves and a 

 root. It is still imbedded in the gametophyte tissue from which it draws 

 its nourishment. 



The ovule, seed-scale, and cone have increased in size in the mean- 

 time, and the seed-scales lose their chlorophyl and become woody. As 

 the supply of water becomes less and less the cone becomes dry, and 

 consequently the young sporophyte stops growing. The cone and seeds 

 are now said to be ripe, so that as the dry seed scales are spread out and 

 blown away, the part of the seed which contains the embryo, is carried 

 with them. As soon as water is again supplied, the embryo again 

 begins to grow, breaking the brittle integument or indusium covering it, 

 and the root is ready to penetrate the soil and carry water to the stem 

 and leaves of the new plant. 



FLOWERS 



The flowers of flowering plants (Fig. 146) consist of cone-like 

 clusters of closed megasporophyls (carpels) above, and microsporophyls 



(stamens) below, subtended 

 by a perianth. The plant on 

 which the flowers grow is the 

 sporophyte. 



The microspores or pollen- 

 cells (Fig. 147), each produce 

 a mature gametophyte which 

 consists of a pollen tube with 

 three nuclei (Fig. 148 B) ; 

 one, the nucleus of the pollen 

 tube itself, and the other two, 

 sperm nuclei. 



The megaspore is retained 

 within the ovule (Fig. 148 A), 

 (megasporangium). A gameto- 

 phyte with a single egg devel- 

 ops within the ovule. After 

 fertilization, the zygote devel- 

 ops into an embryo and an en- 



Fig. 146. Floral Organs. 



A, Orange blossom.. (After Bailey.) 



B, llydrophyllum, cal, lobe of calyx; cor, lobe of 

 corolla; st, stamens; p, pistil. (After Lindley.) 



C, Diagrams of flower, showing face-view and 

 dissection. r, receptacle; se, sepal; pe, petal; st, 

 stamen; pi, pistil; o, ovule. 



The parts of a complete bisexual flower of the 

 higher seed plants (angiosperms) are sepals, petals, doSDerm, to be described 

 stamens, and pistils. The sepals, taken together, 



constitute the calyx; the petals, taken together, con- shortly, while the entire OVllle 

 stitute the corolla. The calyx and corolla are col- 

 lectively known as the floral envelopes, or perianth. 



Many angiospernwus flowers consist of five cir- 

 cles, or whorls, two of which belong to the perianth, 

 two to the stamens, and one to the pistils. The parts 

 of each circle alternate in position with those of the 

 preceding or following one, and all the members of 

 each circle are alike. (From Bergen & Davis "Prin- 

 ciples of Botany," by permission of Ginn & Co., Pub- 

 lishers.) 



becomes covered with one or 

 two coats to form the seed. 

 With proper moisture and soil, 

 the sporophyte escapes from 

 the seed as with the pine. 

 (Fig. 149.) 



