THE HIGHER CRYPTOGAMIA. 369 



remarkable phenomenon, however, is that exhibited by 

 Isoetes IIi/strLv, and Durieui, in the lignification of the 

 masses of cellular tissue of the bases of their leaves,* which 

 varies in the different varieties of these species. The 

 cells remain in the closest connexion, and are thickened in 

 a porous manner by the superposition of dark-brown layers 

 upon the inner Avails, so that, as in Niphobolus chinensts, a 

 stony, closed bark is formed round the stem. By the 

 development of new leaves the lignified portions of the 

 bases of the leaves are pushed more and more outwards 

 and after the death of the herbaceous parts of the leaves 

 form a close spiny covering to the stem which can hardly 

 be cut with the sharpest knife, and is a sad hindrance in 

 the examination of these parts. 



In the three-furrowed species of Isoetes, the end of the 

 stem, which occupies the base of the deep and steep depres- 

 sion of the top of the stem, is a wart of cellular tissue of 

 a much flatter form than in Isoetes lacustris. It grows 

 like that of I lacustris, by continually repeated division 

 of the single apical cell. The nature of the cell-multiplica- 

 tion is, however, essentially different. The septa, an end- 

 less series of which appear in the apical cell, are turned 

 in three different directions. The apical cell has the form 

 of a three-sided pyramid, with the top turned downwards ; 

 the cells of the second degree are produced by the forma- 

 tion of septa successively parallel to each one of the lateral 

 surfaces (PI. LIII, fig. 22). The cells of the second 

 degree form a spiral, winding round the middle point of 

 the primary cell, which spiral, as far as observations have 

 hitherto gone, is ahvays a right-handed one, and becomes a 

 snail-shell-spiral, in consecuience of the fact, that the cells 

 of the second degree from the time of their formation 

 grow by expansion and multiplication in all three direc- 

 tions. 



As far as observations have hitherto gone, all the septa 

 formed in the apical cell and turned in one of the 

 three directions, are at right angles to a plane passing 

 through that indentation of the stem which is nearest 

 to them. Consequently, in one of the most essential 



* A. Brauii, 1. c, pp. 35, 36. 



24 



