350 FERTILISATION AND EMBRYOLOGY 



short (four to six weeks) where the seeds mature in the same 

 year as that in which poUination occurs {e.g. Thuja, Picea), but 

 is very prolonged (twelve to thirteen months) in the Pines, where 

 pollination ensues in May or early June, whilst the seeds are not 

 shed till the autumn of the following 3'ear. In the latter case 

 there is very pronounced growth of the female cones in the 

 interval between pollination and fertilisation (cf. Fig. 200). 



With the resumption of growth the pollen tube penetrates 

 deeper into the nucellus (Fig. 203, p.t.), frequently exhibiting 

 slight lobing, and the two naked male cells (often merely nuclei), 

 resulting from the di\-ision of the body cell, pass down into its 

 apex (Fig. 204, C, o). On reaching an archegonium the neck 

 is crushed, and the pollen tube, breaking open at its tip, discharges 

 its contents into the ovum. Subsequently one of the two male 

 nuclei fuses with the egg-nucleus. In Pin lis the second male 

 nucleus is smaller (Fig. 204, C) and aborts, though in some genera 

 {e.g. of Cupressinea2), where the archegonia occur in groups and 

 the pollen tube discharges into the common archegonial chamber, 

 both the male nuclei, which are here of equal size, may function. 



The fertilised egg, becoming enveloped in a delicate membrane, 

 almost immediately exhibits two successive divisions of its 

 nucleus (Fig. 204, D). The four nuclei thus formed wander to 

 the end of the egg remote from the micropyle, where they become 

 grouped in a single plane. Further division, accompanied by 

 the formation of separating walls, ultimately results in the 

 development of three or four superposed tiers (Fig. 204, E), each 

 usually consisting of four cells and occupying only a small part 

 of the oospore. In this prooiibryo the tier furthest from the 

 micropyle mostly gives rise to the new plant, ^ whilst the ceUs 

 of the adjacent tier elongate very considerabl}- (Fig. 204, F, S.), 

 and form a sitspcnsor (cf. Sclagiiuila, p. 319), wliich carries the 

 developing embryo {em.) down into the middle of the prothallus. 



The cells of the embryonic tier divide repeatedly to form 

 an extensive mass of tissue. At the end away from the micro- 

 pyle a number of lobes soon grow out and form the cotyledons 

 (Fig. 204, G, cot.). These surround the developing plumule {p.), 



' In Pinus and some other genera the cells of the proembryo commonly 

 separate, and as many as eight embryos may be formed, but, as in other 

 conifers, one only reaches maturity. 



