CRYPTOGAMS AIS'D PHANEROGAMS. 



247 



sperm in -which it may rest for a longer or shorter time, and 

 generally is developed to such an extent that it has a distinct 

 primary-root (radicle) and stem (plumule) with one or more 

 embryo-leaves (cotyledons). 



When the oospbere has been fertilised its nucleus sinks down to its lower end, 

 and by repeated division into two, forms four cells lying in one plane (Fig. 253, 

 see base of the left archegoniurn). Three tiers of cells are now formed by trans- 

 verse division of these four. It is the intermediate one of these which elongates 

 and forms the suspensor, or four suspensors, if they separate from each otber, 

 which push the lowermost four cells deep down into the endosperm. It is 

 from these four lower cells that the embryo (or four embryos when the suspen- 



ant 



FIG. 254. Embryo-sac of 

 Cnrex prcecox : syn synergi- 

 d; ; fcb the oosphere; c the 

 central nucleus ; ant the 

 aaiipodal cells. 



FIG. 253. The apex of the nucellus (n) of Abies in longi- 

 tudinal section : c, c the oospheres of the two archegonia ; 

 the embryo-formation has commenced at the bottom of 

 the left archegonium ; s wall of the macrospore ; p pollen- 

 grains ; r pollen-tubes. 



SOTS separate) is developed, but never more than one embryo attains full devel- 

 opment. As several archegonia are contained in one and the same ovule, all of 

 which are capable' of forming embryos, there is the possibility that several 

 embryos may be developed in a seed (po'yembryony), but usually only one 

 embryo attains perfect development. 



At the same time that the embryo is being developed, other 

 changes are taking- place in the ovule, especially in the integument 

 which becomes the shell of the seed (testa'). The endosperm grows, 

 and the embryo-sac supplants the cells of the nucellus. The seed 



