268 morphology 



themselves as male cells. In angiosperms there is only one division, 

 the generative cell producing two male cells. These male cells are vari- 

 able in form, being spherical, lens-shaped (very common), spindle-form, 

 curved, or even spirally twisted. 



Fertilization 



Pollination. — Pollination is the transfer of pollen from stamen to 

 Stigma, and is a necessary antecedent to fertilization. The term ferti- 

 lization is often used when pollination is meant, but no student of mor- 

 phology should confuse the two. In angiosperms pollination is a very 

 extensive subject, for insects as well as wind are agents of transfer; in 

 other words, they are insect-pollinated as well as wind-pollinated. This 

 use of insects, which has developed among the higher angiosperms, has 

 involved a variety of mechanism and of habit that is fairly bewildering ; 

 but the whole subject lies within the domain of ecology (see Part III). 

 It is sufficient to note that in some cases the pollen is transferred to the 

 stigma of its own flower (close pollination), and in other cases to the 

 stigma of another flower (cross pollination). Dioecious plants are nec- 

 essarily cross pollinated. Hybrids may be produced by cross pollinat- 

 ing (or crossing) individuals belonging to different species or varieties. 



Pollen tube. — After the pollen grain lodges on the stigma, the pollen 

 tube is developed and penetrates from the stigma to the embryo sac. 

 This involves penetration of the style, entrance into the ovarian cavity, 

 passage along the wall of the ovarian cavity to the insertion of the ovule, 

 passage along the ovule to the micropyle, passage through the micropyle 

 to the tip of the nucellus, penetration of the tissue of the nucellus over- 

 lying the embryo sac, and finally penetration of the sac wall (fig. 599). 

 The time involved in this journey holds no relation to the distance 

 traversed. For example, in Crocus, with a style 6 to 10 cm. long, the 

 time is one to three days; while in Arum, with a style only 2 to 3 mm. 

 long, the time is five days. The range of time, so far as known, 

 is from a few hours to thirteen months (in certain oaks). These long 

 periods are found among angiosperms which are regarded as primitive 

 (the so-called Amenliferae), and suggest the similar condition among 

 gymnosperms. Among these same primitive angiosperms, also, there 

 are found branching pollen tubes, suggestive of the old haustorial habit 

 of the tube. 



Chalazogamy. — Among the Amentiferae there also occurs the phe- 

 nomenon of chalazogamy, which means that the pollen tube does not enter 



