CONIFERAE: THE SCOTS PINE 





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

 canal-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 

 depth, forming the channel of the neck (Fig. 424, A). The ovum 

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

 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. 



em 



A 



A' •.••-...:•'•.• • •»:•,... -A J.''.vl->*.--.v:Vv-::-V: ■-•**:••..,..„_. 



Fig. 425- 

 Pi>i!<s Laricio, stages of embryology. .4 shows two tiers of four cells at base of 

 the archegonium. B shows three tiers, and the last division proceeding in the owest 

 tier. C shows the tier of suspensors (s) elongating, and carrying forward the lowest 

 embryonic tier. (After Coulter and Chamberlain.) 



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 

 the apex of the prothallus. Meanwhile its generative cell lias divided 

 to form a stalk-cell and a body-cell. The former breaks away from its 

 attachment, and the contents of the grain enter the tube. The body-cell 

 divides during transit to form the two male gametes. Thus provided, 

 the tube enters the neck of an archegonium, and the gametes are 

 transferred into the ovum (Fig. 4-M, O- The nucleus of one of the 



