196 



BOTANY: PRINCIPLES AND PROBLEMS 



ment in such families of irregular-flowered plants as the legumes 

 and orchids, and have long excited the curiosity and admiration 

 of naturalists. 



Fertilization. — Pollination, however, is only a step toward the 

 union of male and female gametes which we know as fertilization 



Fig. llO.^The process of seed-production in a flowering plant. Longitudinal 

 diagrams of flower and fruit, the calyx and corolla solid black; the ov-ule, seed- 

 coats and embryo dotted, and the ovary wall, style and stigma lined. A, young 

 bud, the stamens and the single ovule beginning to develop. B, bud ready to 

 unfold. The embryo-sac within the ovule is fully developed and the egg (below) 

 and double endosperm nucleus (in center) are ready for fertilization. C, fully 

 opened flower. The anthers have burst and pollination has taken place, pollen 

 grains being transferred to the stigma. Two grains have germinated, and the 

 pollen-tube from one of them has penetrated the style, entered the ovary, passed 

 through the micropyle of the ovule and discharged its contents — the two male 

 gametes — into the embryo-sac. Double fertilization is taking place, one male 

 gamete uniting with the egg and the other with the endosperm nucleus. D, 

 ripe fruit. Sepals, petals and stamens have dropped off; the ovary wall has hard- 

 ened into the pericarp; the micropyle has closed; the integuments have become 

 seed coats and the ovule has developed into the seed. The embryo, in the center 

 of the seed, has grown from the fertilized egg, and the endosperm surrounding it 

 (shown in white) from the endosperm nucleus. 



(Fig. 110). Although the pollen grain is a single cell, it is not the 

 male gamete. At about the time of pollination, the nucleus of 

 the grain divides into two, one of which, the tube-nucleus, remains 

 free in the cytoplasm. The other nucleus surrounds itself with a 



