CRYPTOGAMS 



465 



A ligule, in the form of a triangular membrane, is inserted above the fovea. 

 Isoetes thus differs greatly in habit from the other genera, but resembles Selaginella 

 in the development of a ligule. 



The macros porangia are situated on the outer leaves of the rosette ; the micro- 

 sporangia on the inner. Both are traversed by transverse plates of tissue or 

 trabeculae, and are in this way imperfectly divided into a series of chambers. The 

 spores are set free by the decay of the sporangial walls. 



The development of the sexual generation is accomplished in the same way as 

 in Selaginella. The reduced male prothallium (Fig. 430) arises similarly within 

 the spore, by the formation of a 

 small, lenticular, vegetative cell, 

 and a larger cell, the rudiment of a 

 single antheridium. The larger cell 

 divides further into four sterile per- 

 ipheral cells, which completely en- 

 close two central spermatogenous 

 cells. From each of the latter arise, 

 in turn, two spermatozoid mother- 

 cells, four in all, each of which, when 

 liberated by the rupture of the spore 

 wall, gives rise to a single, spirally 

 coiled, multiciliate spermatozoid. 

 The female prothallium (Fig. 431) 

 just as in Selaginella, also remains FIG. 431. Isoetes echinospora. A, Female prothallium ; 

 enclosed within the macrospore, ar > archegonium ; o, egg-cell. B, C, development 

 and is incapable of independent of the archegonium from a. superficial cell ; A neck- 



cells ; hk, neck canal-cell ; h, ventral canal-cell ; 



growth. It shows similarly an ap- 0> egg . ce ii. (After CAMPBELL, x 250.) 

 proach to the Conifers, in that the 



nucleus first divides into numerous, parietal daughter-nuclei before the gradual 

 formation of the cell walls, which takes place from the apex of the spore to the 

 base. As a result of this process the whole spore becomes filled with a pro- 

 thallium, at the apex of which the archegonia are developed. The embryo has 

 no suspensor. In the structure of the embryo and of the spermatozoids Isoetes 

 differs from the other Lycopodineae, and resembles in some respects the euspor- 

 angiate Ferns. 



The Fossil Cryptogams ( 123 ) 



The remains of cryptogamic plants of former geological periods afford no 

 evidence as to the phylogenetic relations of the classes of Thallophyta and 

 Bryophyta. Intermediate forms between Algae and Bryophyta and between the 

 latter group and the Fteridophyta are, as yet, unknown. On the other hand 

 Phytopalseontology has made us acquainted with interesting, long-extinct types 

 of Pteridophytes, which serve to'complete the classification of the existing Ferns, 

 Horse-tails, and Club-mosses, and in part afford a transition from the Ferns to 

 the Gymnosperms. 



I. The great majority of Thallophytes are, on account of their delicate structure, 

 not adapted to be preserved in the fossil condition. The absence of remains of 

 many classes of Thallophytes is therefore no ground for concluding that they did 

 not exist in earlier periods. Even in the Silurian rocks remains of Algae are 

 found, though they do not permit of their relationship being determined. The 

 remains of calcareous Algae belonging to the Siphonales are, owing to their good 



2 H 



