THE SPRUCE FIR 275 



IB ready for fertilisation, a very small lens- shaped cell 

 is cut off from the venter at its upper end. This is 

 called the ventral canal cell (see Fig. 113, A, cl). The 

 immensely larger lower cell which remains after this 

 final division is the ovum itself, which will give rise to 

 the embryo after fertilisation. The ova in the Fir and 

 in most Gynmosperms are of relatively very large size, 

 so as to be easily visible to the naked eye. The large 

 scale of the whole reproductive apparatus at the time 

 of fertilisation is convenient for observation, but is some- 

 times puzzling when we have become accustomed to the 

 minute size of the corresponding organs in Angiosperms. 

 Fig 113, A, shows the upper part of the embryo-sac 

 with two archegonia buried in the prothallus. 



Each ovum has a large nucleus, and is densely 

 filled with protoplasm, in which there are numerous 

 vacuoles, often bearing a deceptive resemblance to 

 nuclei. 



The neck of the archegonium, as seen in longitudinal 

 section, consists of two to four tiers of cells (Fig. 113, 

 A and C). When seen in surface view (Fig. 113, B\ 

 it appears as a rosette of six or eightt'cells, so that the 

 total number of cells in the neck may vary from 

 twelve to thirty-two. 



We see, then, that the arrangements within the 

 embryo-sac at the time of fertilisation are totally 

 different in Gyrnnosperms from those in all other 

 flowering plants. On the other hand, they agree 

 closely with the corresponding structures in some of 

 the higher Cryptogams. We must now consider the 

 processes of pollination and fertilisation. 



