422 



BOTANY 



remains in the prothallium the young plant continues united to the 

 spore, and presents the appearance of a phanerogamic seedling with 

 the seed still attached (Fig. 351, B). 



The second genus of this order, Isoetes, the Quillwort, comprises perennial plants, 

 growing either on damp soil or submerged in water. The stem is short and 

 tuberous, terminating below in a tuft of dichotomously branching roots, and above 



in a thick rosette of long, stiff awl -shaped 

 leaves (Kg. 356). The leaves are penetrated 

 longitudinally by four air -passages, and ex- 

 pand at the base into a broad sheath. On the 

 inner side of the leaves, above their point 

 of insertion, is an elongated pit, the fovea, 

 containing a. large sessile sporangium. 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 macrosporangia are situated on the 

 outer leaves of the rosette ; the microsporangia 

 on the inner. Both are traversed by trans- 

 verse plates of tissue or trabecule, 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 Selagi- 

 nella. The reduced male prothallium arises 

 similarly within the spore, by the formation 

 of a small, lenticular, vegetative cell, and a 

 s Icumstru. Q nat. size.) larger cell, the rudiment of a single anther- 

 idium. The larger cell divides further into 

 four sterile peripheral cells, which completely enclose 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, just as 

 in Selaginella, also remains enclosed within the macrospore, and is incapable 

 of independent growth. It shows similarly an approach 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 an endosperm- 

 like prothallium, at the apex of which the archegonia are developed. The 

 embryo has no suspensor, and is similar in many respects to the embryo of the 

 Monocotyledons, to which the mature plant also bears a strong resemblance. 



The Lepidodendreae, an extinct family of arborescent Pteridophytes occurring 

 chiefly in the Carboniferous period, belong also to the Lycopodinae. They were 

 usually sparingly branched, either dichotomously or sympodially, and provided 

 with linear or lanceolate leaves, thus resembling, to a certain extent, gigantically 

 developed Club Mosses. Their stems increased in thickness by secondary growth, 

 and were covered with cushion-like areas, showing the scars of the fallen leaves. 



Fig. 356. ■ 



