26.] CHAPTER I. THE CELL. 117 



substance (presumably kinoplasm) of the receptive spot is ejected into the 

 water; the oosphere is now ready for fertilisation. In the archegonium of 

 Bryopbyta, Pteridopbyta, and most Gymnosperms, the " central cell " under- 

 goes division into two ; a smaller cell, the ventral canal-cell, next the neck of 

 the archegonium ; and a larger lower cell which rounds itself off and con- 

 stitutes the oosphere. 



This process of cell-formation is closely followed in some ccenocytic plants. 

 Thus the zoogonidia.of Vaucheria are developed in essentially the same manner 

 as those of CEdogonium, the differences being those which are necessarily in- 

 volved by the fact that the one is developed from a cell, and the other from a 

 coanocyte. In Vaucberia, the many nuclei travel, with the kinoplasm, to the 

 periphery of the developing zoogonidium ; the kinoplasm, instead of a lirr-ited 

 area as in OEdogonium, forms a layer over the whole surface in which lie the 

 numerous nuclei, and from which many pairs of cilia are developed, a pair 

 opposite each nucleus (Fig. 75). 



The development of tbe oosphere' in Vaucheria resembles in its main feature* 

 that described for CEdogonium ; but here the mass of protoplasm encloses many 

 nuclei at first, though it appears that there is but one nucleus in the mature 

 oosphere, which has awell-defined receptive spot. In Peronospora likewise,, 

 where it is formed from a part only of the protoplasm, tbe single oosphere con- 

 tains, at first, many nuclei, but only one when mature. The reduction in th 

 number of the nuclei in these cases appears to be brought about by fusion. 



In the foregoing cases of cell -formation with the development 

 of a single new cell, the cell formed is a primordial cell destitute 

 of a cell-wall, at least, for a time. In other cases the cells sur- 

 round themselves at an early stage with a proper wall of their 

 own : this takes place in connection with the development of the 

 spores in the Bryophyta, Pteridophyta, and Phanerogams, 



In illustration of this mode of cell-formation, a brief general account of the 

 development of spores in the higher plants may be given. The- mother-cells of 

 the spores undergo division in a manner described under the head of cell-divi- 

 sion (p. 125), so as to give rise to four special mother-cells,, lying either all in one 

 plane (e.g., some Pteridophyta, Monocotyledons), or arranged tetrahedrally in a 

 pyramid (some Bryophyta and Pteridophyta, most Gymnosperms, Dicotyledons). 

 In each of these special mother-cells a single spore (resp. pollen-grain) is de- 

 veloped. The ectoplasmic layer of the cytoplasm becomes converted into a 

 membrane, the exine (or exospore), which, though at first consisting of cellulose, 

 becomes eventually cuticularised, and acquires a more or less elaborate struc- 

 ture. In a few cases this is the only coat of the spore ; but, as a rule, a second 

 membrane, the inline (or endospore), is eventually formed internally to the 

 exiue, from the ectoplasmic layer of the cytoplasm : it consists always of cellu- 

 lose. The walls of the special mother-cells become disorganised, and the spores 

 are set free. 



The development of the proper coats of the spores, as also their subsequent 

 growth in surface and in thickness, is effected solely by means of the cytoplasm 



