CONIFERAE: THE SCOTS PINE 311 



lese originates from a single superficial cell of the prothallus, and 

 consists of a large nucleated ovum, with a small lenticular ventral- 



inal-cell lying above it, 'which is cut off from the ovum shortly before 

 fertilisation. Covering this is a group of cells two or more tiers in 

 lepth, forming the channel of the neck (Fig. 254, A). The ovum 

 jf Pinus, as is not uncommonly the case in Gymnosperms, is large 



n 



FIG. 253. 



Median longitudinal section of an ovule of Pitea excelsa at time of fertilisation. 

 = embryo-sac filled with tissue of the female prothallus. a = archegonium, 

 showing venter (a) and neck (c). n = nucleus of ovum. c=nucellus. p= pollen- 

 grains. <=pollen-tube. t = integument. ( x 9.) (After Strasburger.) 



enough to be seen with the naked eye. The archegonia lie in a slight 

 depression of the surface of the prothallus. The last step before 

 fertilisation is the collapse of the ventral-canal-cell, which takes no 

 direct part in propagation. 



In the Scots Pine a whole year elapses between the pollination of 

 the young pink cone and the act of fertilisation. But in Conifers 

 generally the times are different. With or without a lengthened 

 interval each pollen-grain, germinating on the apex of the nucellus, 

 forms a pollen-tube, which penetrates the nucellus, passing towards 



