SPERMATOPHYTA 



391 



generative nucleus and tube nucleus, the former becoming organized as a 

 small, naked generative cell lying within the larger tube cell (Fig. 334). 

 The generative cell is usually elliptical, lenticular, or spindle-shaped. 

 The tube nucleus is generally large, with a large nucleolus and little 

 chromatin. The generative nucleus is usually smaller, with a small 

 nucleolus or none, and with considerable chromatin. The generative 

 cell gives rise directly to two male cells, dividing either within the pollen 

 grain or, somew^hat more frequently, in the pollen tube. The male cells 

 show considerable variation in form but are 

 never ciliated. In most angiosperms the male 

 cells remain intact, while in some the mem- 

 brane around each seems to disappear, leav- 

 ing their nuclei free. The male nuclei often 

 become vermiform, especially after entering 

 the embryo sac. 



Fertilization. As in gymnosperms, pollina- 

 tion must precede fertilization but, because 

 the ovules of angiosperms are enclosed in an 

 ovary, the pollen grains cannot come in con- 

 tact with them. Pollen is transferred by vari- 

 ous agencies from the anther to the receptive 

 surface of the style (the stigma), where it 

 germinates, putting forth a long pollen tube 

 that grows down the inside of the style and 

 into the cavity of the ovary. Branching 

 pollen tubes, characteristic of gymnosperms, 

 are found in only a few angiosperms, notably 

 among members of the amentiferous orders. 



Where there is a stylar canal, the pollen tube usually grows down 

 through it, but where the style is solid, as is more commonly the case, the 

 tube secretes enzymes that digest a passageway to the ovary. The 

 nucleus and cytoplasm of the tube cell, as well as the generative cell, pass 

 down the pollen tube as it develops. The tube nucleus usually lies at the 

 tip of the advancing pollen tube and apparently is concerned with its 

 development. While the tube is developing, or frequently before the 

 pollen grain is shed, the generative cell gives rise to two male cells. 



Upon reaching the cavity of the ovary, the pollen tube grows along the 

 ovary wall until it reaches one of the ovules, which it then enters, ordi- 

 narily through the micropyle. After penetrating the intervening nucellar 

 tissue, the tip of the pollen tube ruptures and its contents are discharged 

 into the embryo sac. The tube nucleus soon disintegrates, but both male 

 cells (or male nuclei, as the case may be) enter the embryo sac. One of 

 the male nuclei penetrates the egg and unites with the female nucleus, thus 



Fig. 334. Section of a pollen 

 grain of lily {Lilium auratum) 

 in the shedding condition, 

 X750. The smaller, naked 

 generative cell lies within the 

 larger tube cell, each having 

 its own nucleus. 



