52 VASCULAR CRYPTOGAMS 



from a hypodermal layer of cells. In the formation of the microsporange 

 the archespore-cells elongate in a direction at right angles to the surface, 

 and divide by transverse walls. Some of these rows of cells are then 

 arrested in their growth, lose their abundant protoplasm, and divide 

 into elongated tabular cells constituting the trabecules, which cross the 

 sporange from the dorsal to the ventral side. The remaining cells 

 develop into the mother-cells of the microspores, an external layer 

 having been previously separated as tapetal cells. In the development 

 of the megasporange the processes are the same as far as regards the 

 formation of the tapetal cells and the trabecules ; the mature sporange 

 may contain either four or a much larger number of megaspores. The 

 mode of development of the megaspores presents perhaps the closest 

 analogy to that of the secondary embryo-sacs of Gymnosperms that 

 occurs in any order of Vascular Cryptogams ; and the same remark 

 applies to the formation of the microsporanges and pollen-sacs. Both 

 kinds of sporange are indehiscent, the spores escaping only by the decay 

 of the tissue. 



In both kinds of spore the epispore is frequently granulated, tuber- 

 culate, or echinate ; and in some species there are two kinds of microspore 

 differing from one another in this respect. 



One or two species of Isoetes display the phenomenon of apogamy in 

 various degrees. In extreme cases the formation of the megasporange 

 is arrested at a very early stage, and its place supplied by a vegetative 

 shoot, which becomes detached and develops into an independent plant. 



The number of species of Isoetes is about fifty, the greater part 

 inhabitants of the warmer portions of the globe. They somewhat 

 resemble Pilularia in general habit. Some species are aquatic and 

 entirely or partially submerged, other paludose, and a very few terrestrial ; 

 and they present corresponding differences in the structure of their tissue, 

 presence of stomates, &c. 



Literature. 



Von Mohl — (Stem of Isoetes) Linneea, 1840, p. 181. 



Braun — Ueber Isoetes, Monber. Berlin Akad. Wiss., 1863. 



Hofmeister — Entwick. d. Isoetes lacustris, Abhandl. Sachs. Gesell. Wiss., 1865. 



PfefFer— Entwick. d. Keims Selaginella, in Hanstein's Hot. Abhandl., iv., 1871. 



Tchistchakoff— (Isoetes) Nuov. Giorn. Bot. Ital. , 1873, P- 207. 



Bruchmann — Wurzeln v. Lycopodium u. Isoetes, 1874. 



Hegelmaier — Bot. Zeit., 1874, p. 481. 



Goebel — (Apogamy of Isoetes) Bot. Zeit., 1879, p. i. 



Mer— (Sporange of Isoetes) Compt. Rend., xlii., 1881, p. 310 ; and Bull. Soc. Boti. 



France, 1881, pp. 72, 109. 

 Kienitz-Gerloff — (Embryo of Isoetes) Bot. Zeit., 1881, pp. 761, 785. 

 Vines— (Isoetes) Annals of Botany, vol. ii., 1888, p. 117. 



