THE ARCHEGONIAT^E 



199 



Sporophyte. The sporophyte (Fig. 160) shows a very different 

 degree of development 

 among the Archegoni- 

 ates. In its simplest 

 form (e.g. Riccia) it is 

 a globular body which 

 is almost entirely com- 

 posed of sporogenous 

 tissue. In the Ferns, 

 spore-production is 

 largely subordinated to 

 the vegetative exist- 

 ence of the sporophyte, 

 which becomes a large, 

 leafy plant. Sooner or 

 later the sporophyte 

 develops a special spo- 

 rogenous tissue, each 



cell of which, by a fur- FlQ m _ A> Sphserocarpus cristatus; median sec- 

 ther division into four tion of young sporophyte (x'2'25), the nucleated 

 parts (Fig. 161), pro- cells constitute the archesporium ; /, foot. B, C, 

 HUPPS thp sr>orps which Fossombronia longiseta, two ripe spores, B, and an 



elater, C(x225). 

 are very similar in 



structure throughout the group. The sporogenous tissue (Arche- 

 sporium) may be developed from the inner, tissue of the sporophyte, 

 or there may be a special organ, the sporangium, in which the 

 spores arise. 



Spore-formation. The development of the spores among the 

 Archegoniates is very uniform, and is one - of the strongest proofs 

 of a common origin for all of them. The sporogenous cells arise 

 from a single archesporial cell, or from 'a group of these. Each 

 sporogenous cell contains a large nucleus which divides twice. 

 The divisions may be followed at once by a division-wall, but more 

 often the four daughter-nuclei lie free in the cytoplasm of the mother- 

 cell. Division-walls are then formed simultaneously between the 

 nuclei, and the resulting four spores are tetrahedral in form. The 

 ripe spores usually contain a large amount of starch, oil, or albumi- 

 nous reserve-food, and are protected by a heavy outer spore-coat, or 

 Perinium, marked with characteristic thickenings. 



Reduction of Chromosomes. It has been ascertained that, in some 

 cases at least (e.g. Osmunda regalis, Pallavicinia decipiens), the num- 

 ber of chromosomes in the nuclei of the sporophyte is double that 

 of the gametophytic nuclei. The reduction takes place in the last 

 division of the archesporial cells, which results in the spore mother- 

 cells. 



