THE HIGHER CRIPTOGAMIA. 367 



clothed with an episporium (PI. LIII, fig. 18), of the nature 

 described at the commencement of this chapter, and be- 

 comes free by the dissolution of the special-mother-cell, 

 some time before the rupture of the walls of the sporangium 

 permits the escape of the spores, whose outward ap- 

 pearance, when ripe, exhibits no farther change. The 

 special-mother-cells of the large spores, which have always a 

 tetrahedral arrangement, remain for a longer time united in 

 fours even until after the formation of the exosporium.* 



Alexander Braun's observations have shown that amongst 

 the species of Isoetes, those at least of the old world, 

 Isoetes lacustris is the only one which has only one furrow 

 on the underside of the stem. All others, the South-West 

 European and the North African species, have three, in 

 exceptional cases, four, deep indentations of the under 

 surface of the principal axis. The new roots break forth 

 from the base of the deep furrow ; in the species which 

 inhabit dry localities these roots are far more numerous than 

 in Isoetes lacustris. Even in the three-furrowed species 

 the form and structure of the wood always corresponds 

 exactly with the number and position of the furrows of the 

 bark. The lower portion of the wood is three-armed ; it 

 consists of three laterally-flattened arched masses of wood, 

 meeting at angles of 120, and formed out of the closely 

 crowded rudiments of the vascular bundles of the roots, 

 and of the tissue between these vascular bundles, part of 

 which tissue is changed into spiral cells, and part remains 

 thin-walled. Where the number of roots is much greater 

 than in Isoetes lacustris, as is the case especially with 

 I. Hystrix and I, Durieui, the development of the lower 

 part of the wood also is unequal. Each of the three arms 

 of the lower half of the wood meets one of the deep cortical 

 furrows (PI. LIII, fig. 20). The newly formed roots originate 

 the lower arched margin of the plate of wood ; they bend 

 in an arcuate manner, and breaking through the side-walls 

 of the furrow, make their appearance at the deepest part 

 of the latter. Many cycles of roots are developed in each 

 vegative period j I have seen as many as eight in old strong 

 plants of Isoetes Ilystrino. In the greater number of the 



* Wahlenberg, 'Flora Lapponica,' PI. xxvi, fig. 1, K. 



