THE HIGHER CRYPTOGAMIA. 383 



placed in the middle of a hollow cylindrical air-cavity — 

 with the remaining tissue of the stem (PL LVII, fig. 11). 

 These cells afterwards divide by a transverse septum, once 

 at least in those species in which the air-cavities have a 

 proportionably narrow diameter, and several times in those 

 species where the air-cavities attain a considerable develop- 

 ment, as in Selaginella helvetica. 



A string of cells passing from the vascular bundles of 

 the stem through the thickened longitudinal axis of each of 

 the neighbouring leaves, becomes transformed into a vas- 

 cular bundle whose nature and structure resembles in its 

 essential features those of the stem. The formation of the 

 vascular bundles always commences earlier in the stem 

 than in the leaves ; it progresses from the former towards 

 the latter (PL LVI, fig. 11). 



Adventitious roots spring from the forks of the stem ; in 

 S. denticulata and helvetica they spring from each fork, after 

 the commencement of the final longitudinal expansion of 

 the stem. In S. Martensi and Galeottii, and still more so 

 in S. viticulosa the upper ramifications of the upright 

 shoots, which have a tendency to produce fruit, are apt to 

 be devoid of roots. The root is always situated in the 

 angle of the primary leaf, which in the bud apparently 

 occupies the middle of the forking end of the stem. It 

 originates on the outer side of the transverse junction which 

 in the fork of the stem unites the two vascular bundles 

 (PL LIV, fig. 3). The mode of cell-multiplication in the 

 growing tip of the root exactly resembles that in the Equi- 

 setacese and Polypodiacese ; the examination of it is much 

 more difficult than in the latter plants on account of the 

 smallness of the cells. The roots of the Selaginellse, like 

 those of the Lycopodiacese in general, are usually several 

 times branched ; they exhibit the most regular furcations 

 in two directions diverging from one another at about 90°. 

 The first fork of a root is usually parallel to the surfaces 

 of the leaf in whose axil it has originated, and the second 

 at right angles to those surfaces.* The outer side of the 

 root-cap is often clothed with long papillae, which, as the 



* The relation is very manifest in the roots of S. Galeottii, and Martensi 

 which ramify frequently high above the ground. 



