486 



BOTANY 



sporangium consists of the nucellus and an integument. The micropyle forms 

 iin open canal above the tip of the nucellus. At the period during which the male 

 cones are shedding their pollen, the macrosporophylls 

 become slightly separated from one another so that the 

 wind-borne pollen-grains can readily enter. A more or 

 less extensive cavity (POLLEN-CHAMBER) has by this tiim 

 been formed at the apex of the nucellus, while the disin- 

 tegrated cells, together perhaps with fluid excreted from 

 the surrounding cells of the nucellus, form a sticky mass 

 which fills the mieropylar canal and forms a drop at it- 



Kli;. 447. CarjM'1 of ' >cs re- 

 niltita. (After SACHS, re- 

 duced.) 



Fin. 44S. Stamen of 'i/cn* i-ii-i-iiiiilia, from U<-lo\v. (After RICHARD.) 



entrance. The pollen-grains reach this drop and, with the gradual drying up ot 

 the fluid, are drawn through the mieropylar canal into the pollen - chamber. 

 During the development of the pollen-tube and the formation of the motile sper- 

 matozoids, which have been found in all the Cycads yet investigated, the embryo- 

 sac filled with the prothallial tissue is increasing 

 in size within the nucellus. As it crushes the 

 tissue of the upper portion of the nucellus it 

 approaches the base of the pollen-chamber. At 

 the apex of the embryo-sac are found the arche- 

 gonia, usually four in number, and separated 

 from one another by some layers of cells. Each 

 archegonium has a neck, and ultimately cuts 

 off a canal-cell. The archegonia are situated at 

 the base of a depression in the prothallium, the 

 archegonial chamber, which is. about 1 mm. in 

 depth and 2 mm. across. The jiollen-tubes grow 

 into this depression and liberate their sperma- 

 tozoids together with a drop of watery fluid in 

 which they swim. Since the bursting of the 



pollen-tubes is brought about by their contact 

 Fio. 440. CmUOKMria robiuto. Macro- * , , . . ,. . , 



s I( oro,,l,yll with two macrosporania. wlth the projecting cells ot tin- neck, the sp-rn,,i- 

 (After GOEBEL.) tozoids cannot easily lose their way to the egg- 



cell. They require, however, to narrow consider- 

 ably in order to pass through the space between the neck cells. The nucleus 

 of the fertilised ovum soon divides, and the daughter-nuclei continue to divide 

 rapidly, until after the eighth division there are about 256 free nuclei within 

 the cell. These are crowded towards the lower end of the pro-embryo, where cell- 

 formation commences around them. The embryo is forced into the endosperm 



