REPRODUCTION IN PLANTS 273 



destination solely by chance. The grasses are the largest group of flowering 

 plants making use of this method. 



Fertilization. Pollination is not the same thing as fertilization, though 

 we commonly think of it thus. It merely brings the male gametophytes 

 (pollen grains) into a position which makes fertilization possible. As soon 

 as it becomes attached to the stigma, each pollen grain begins to grow. It 

 sends a slender, delicate-walled pollen tube down through the tissues of 

 the style to the ovary; there the tube turns in and grows up through the 



Fig. 17.18. The yellow-fringed orchid (Habenaria ciliata, family Orchidaceae) and its 

 adaptations for insect pollination. Left, the entire inflorescence; right, a much enlarged 

 view of a single flower from in front. The two large, down-pointing prongs are the stamens. 

 The two erect, club-shaped bodies attached to their tips are the modified anthers (pollinia), 

 containing masses of waxy pollen. Where each pollinium is attached to the stamen, it has 

 a sticky gland. When an insect enters the flower, the two pollinia are glued to its head or 

 thorax; during the flight to the next flower they bend forward and assume a position in 

 which they will touch the stigma. (Photos by Prof. E. B. Mains.) 



mouth of an ovule until it makes contact with the ovum and the female 

 gametophyte. As the tube lengthens, the three pollen nuclei move for- 

 ward near its tip. One, the tube nucleus, seems to control the growth of 

 the pollen tube. As soon as the tube reaches the egg and the gametophyte, 

 one of the two sperm nuclei enters the egg and fuses with the egg nucleus, 

 fertilizing it and forming a diploid zygote. The other sperm nucleus enters 

 the large endosperm cell and fuses with the double endosperm nucleus, 

 producing a triploid condition in that cell and its descendants. 



Seeds and fruits. Immediately after fertilization the zygote begins 

 to develop into an embryo within the ovule. The petals of the flower drop, 



