244 CRYPTOGAMS AND PHANEROGAMS. 



same time an indication of sex is found in the pro thai Ha, which 

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

 majority of cases among the isosporous Vascular Cryptogams, the 

 sexual generation pro thallium 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 (Ophioglossacece, Lycopodium annotinum)the prothallium 

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

 is relatively large. In the heterosporous 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. Among the Hydropterides the con- 

 tents of the microspore divide into three cells, from the lower one 

 of which a small lenticular cell is cut off, while from the two 

 upper or from the upper one only (as in Azolld) a very simple 

 antheridium is developed. The male prothallium is thus reduced 

 in this group to a very few cells. In the heterosporous Lycopo- 

 diacese also, the prothallium is represented by one small lenticular 

 cell, and only one antheridium is present which gives rise in 

 Selaginella to a large number of spermatozoids (Fig. 233), but in 

 Isoetes to only four. The spermatozoids are set free through 

 rupture of the microspore- wall, by the swelling of the wall of the 

 antheridium. When, however, the microspores are not liberated 

 from the sporangium (Salvinia), the upper cell of the prothallium 

 elongates and perforates the walls of both the microspore and the 

 sporangium (Fig. 214) in order to protrude the antheridium, thus 

 resembling the protrusion of the pollen-tube in the Phanerogams. 



In the Phanerogams, the microspores are termed pollen-grains. 



In the GYMNOSPERMS the male prothallium is represented by a 

 small cell cut off laterally by a curved wall from the large pollen- 

 cell (Figs. 250, 267), the vegetative cell of the prothallium. The large 

 cell, by the protrusion of the endospore, grows out into a tubular 

 body known as the pollen-tube (Fig. 250) into which the vegetative' 

 nucleus passes. The small cell, corresponding to the rudiment of 

 the antheridium, divides into 1, 2, or 3 small cells, the innermost, 

 or when only one is present this single one, being the antheridial 

 cell. This divides, either after cutting off an inner stalk-cell or 

 directly, into two generative cells, which in Cycadacece and Gink- 

 goaccce are liberated from the pollen-tube as spermatozoids, or,. 

 as in the higher Grymnosperms, pass into its anterior end as naked 



