312 



BOTANY OF THE LIVING PLANT 



the apex of the prothallus (Fig. 254, C). Meanwhile its generative cell 

 has 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. The nucleus of one of the 

 gametes fuses with the nucleus of the ovum. The result of the fusion 



FIG. 254- 



Picea vulgaris. A = longitudinal section through apex of female prothallus, and 

 one archegonium. = neck. v.cc= ventral canal cell. <w=ovum. B neck seen 

 from above. C= entry of pollen- tube with gametes into the canal of the arche- 

 gonium. (A x 100 ; B, C x 250.) (After Strasburger.) 



is the zygote. Both the ovum and the male gamete were haploid, 

 and the consequence of their fusion is to initiate the new diploid phase, 

 which forms the embryo. The process is variable in different genera of 

 Conifers, and rather complicated. 



In Pinus and its allies the nucleus of the zygote divides at once, first into 

 two and then into four. The resulting nuclei sink to the base of the egg., 

 lying in a single plane. Divisions follow to form four tiers consisting of four cells 

 each (Fig. 255). Of these the lowest but one elongate as the suspensors (s) ; the 

 lowest form the embryonic tier (e). In the Pines these may either form together 

 a single embryo ; or they may separate, each borne on its own suspensor, and 

 so four embryos may result from one fertilisation. As there are several 

 archegonia, and each may be fertilised, a high degree of polyembryony is 

 possible. As a rule one embryo in each ovule secures the ascendency over the 



