538 



BOTANY OF THE LIVING PLANT 



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

 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 detail of embryology 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. 425). 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 

 others, and the rest are absorbed. It has been calculated that in the Scots 

 Pine only about one per cent, of the potential embryos are matured. 



The maturing embryo, borne on the end of its elongating suspensor, 

 is thrust downwards into the substance of the endosperm. With 



less regular segmentation than that 

 in Angiosperms it matures into the 

 germ, which in the ripe Pine-Seed is 

 cylindrical in form. It is terminated 

 by an apical cone, round which coty- 

 ledons, varying to the number of 

 fifteen, are arranged in a ring. The 

 radicle is massive, with the large root- 

 cap characteristic of Gymnosperms. 

 The female prothallus persists as the 

 nutritive endosperm ; the nucellus is 

 crushed between the enlarging endo- 

 sperm and the hardening seed-coat. 

 The seed is thus albuminous, which is 

 the case for all Gymnosperms, a fact 

 which suggests that this is the 

 primitive state for the seed generally. 

 Its parts when mature correspond in 

 nature and position to those of the 

 seed in Angiosperms (Fig. 426, I.). 

 A period of rest is followed by 

 germination of the seed. The 

 embryo, drawing upon the nutritive 

 endosperm, enlarges, and the radicle 



I. Median 



Fig. 426. 



Pine seed and germination, 

 section of seed ; y=micropolar end. 

 11. germination. III. Ditto later; s = 

 seed-coat; e=endosperm; c = cotyledons. 

 a> = primary root; Ac=hypocotyl. (After 

 Sachs.) 



