244 CRYPTOGAMS AND PHANEROGAMS. 



same time an indication of sex is found in the prothallia, which 

 finds expression in the forms of the spores themselves. In the 

 majority of cases among the isosporons Vascular Cryptogams, the 

 sexual generation prothallium is a green, leafy expansion which 

 can sustain itself by the assimilation of carbonic acid, and by the 

 absorption of nutriment from the soil by means of root-hairs. In 

 some plants ( Ophwglossacece, Lycopodium annotinum)tihe prothallium 

 is a subterranean, pale, tubercular body, but in these instances it 

 is relatively large. In the heterosporons Vascular Cryptogams and 

 in the Phanerogams, the prothallium is much more reduced, both 

 as regards its size, and also with respect to the number and struc- 

 ture of the antheridia and archegonia. 



1. The Microspores. The PEOTHALLIUM in all Vascular 

 Cryptogams which have unequal spores, consists of a single, vege- 

 tative (barren) cell, which plays a very unimportant part in the life 

 of the prothallium (Fig. 233 A*). In Stilriiu'n it is somewhat elon- 

 gated and tubular, because it must break through the sporangium 

 ( Fiy. '214) ; but in other cases it is very small and lenticular. In all 

 these plants only one antheridium is formed. In Salvinia it con- 

 sists of 2 cells whose walls are ruptured in order that the spermato- 

 zoids may be liberated (Fig. 214 7>, C). In Marsilia, Isoiites, and 

 SelaijiiK'Ud the prothallium does not leave the spore, and consists 

 for the most part of primordial spermatozoid-mother-cells without 

 ci-lf -tcnU, which on germination are ejected so that the spei-mato- 

 zoids are set free. 



In the Phanerogams, the microspores have from olden times 

 leen. termed pollen-grains. 



In the GYMNOSi'KRJis the prothallium is reduced to 1, 2 or 3 small 



cells, placed on one side of the mature pollen-grain (at the top 



in Fig. 250 I, 77, and in Fig. 267 N) and which do not play any 



part in the germination of the pollen-grain. The antheridium is 



represented by the remaining portions of the interior of the pollen- 



<_:rain. that is, it consists of a large cell with a nucleus which does 



not even go so far as the antheridium of SelagineUa and become 



divided into spermatozoid-mother-cells without cell-wall, for even 



these cells are not formed. The unicellular antheridium grows, 



on tin- germination of the pollen-grain, into a tubular body known 



as the pollen-tube, formed from the inner wall of the pollen-grain 



I Fig. -J ">"), which works its way down the micropyle to the 



oosphere. The FIT! ilisation takes place by diosmosis through the 



cell-wall, and consists here also of the coalescence of the nucleus 



