SECT. I 



CRYPTOGAMS 



469 



presents the appearance of a phanerogamic seedling with the seed still attached 

 (Fig. 413, B). 



Order 3. Isoetaceae (^^') 



The isolated genus Isoctcs must be regarded as a persistent branch of an ancient 

 group of plants, which in earlier geological periods was more richly represented. 

 It also exhibits some affinities with the eusporangiate Ferns. The species of 

 Isoetes are perennial plants, growing either on damp soil or submerged in water. 

 The stem is short and tuberous, rarely dichotomously branched, terminating below 

 in a tuft of dichotomously branching roots, and above in a thick rosette of long, 

 stiff, awl-shaped leaves (Fig. 418). The stem is characterised by a secondary 



growth in thickness by means of a cambium ; 



this produces to the outer side cortex (without 



phloem) and to the inner side secondary xylem. 



The leaves are traversed longitudinally by 



Fio. 418.^Jsoetes lacustris. (J iiat. size) 



Fio. 419. — A-F, Isoetes setacea (x 640). A, microspore 

 seen from the side. B-D, segmentation of the spore ; 

 p, prothallial cell ; w, the four cells of the wall ; 

 s, spermatogenous cells. E, the four spermatozoid 

 mother-cells are surrounded by the disorganised cells 

 of the wall ; surface view. F, the same in side view. 

 G, Isoetes Malinverninnn, spermatozoid. (x 780. 

 After Belajeff.) 



four air -passages, and expand 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 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. 419) arises similarly within 

 the spore, by the formation of a small, lenticular, vegetative cell (ji), and a larger 

 cell, the rudiment of a single antheridium. The larger cell divides further into 



