482 VASCULAR CRYPTOGAMS. 



of long-continued growth both in length and in circumference, which accounts for the 

 intervals between the leaves of old stems and for considerable thickness of the stems 

 themselves, a fact which is worthy of investigation not only with reference to these 

 plants, but also to the Lycopodieae and many Ferns, It is a striking peculiarity of the 

 Selaginelleae that the ground-tissue (as also in the stem of Mosses) presents none of 

 the usual small intercellular spaces, a result probably of the prosenchymatous arrange- 

 ment of the cells. This is compensated for by the development of a large air-cavity, 

 which everywhere surrounds each fibro-vascular bundle of the stem (Figs. 340 and 341). 

 This cavity is traversed by transverse rows of cells forming trabeculae attached to the 

 bundle : if the cells are somewhat rounded, the bundle appears to be surrounded by 

 a loose spongy parenchyma (Fig. 339), which is sharply defined from the firm compact 

 ground-tissue. The ground-tissue of the leaf is a loose spongy parenchyma containing 



Fig. 340.— Tran sverse section of the stem oi Selagiitella inaquali/olia (X igo). 



chlorophyll ; in small species with thin leaves this tissue is developed only round the 

 single fibro-vascular bundle traversing the leaf, so that at the margins the epidermis of 

 the upper and that of the under surface come into contact. 



The Fibro-'vasciilar Bundles, one or more of which traverse the stem, are cauline, 

 like those of the Lycopodieae. They can be traced in the form of procambium beyond 

 the youngest leaves up into the apex of the stem to close beneath the apical cell. The 

 separate bundles coming from the leaves become united with the cauline bundles in 

 these plants, as in the Lycopodieae, only at a later period. In their composition the 

 fibro-vascular bundles resemble those of the true Ferns. They have usually an elon- 

 gated elliptical form. The xylem is central, consisting for the most part of scalariform 

 tracheides, and it is surrounded by the thin-walled phloem (Figs. 339, 340). The very 

 narrow spiral vessels (Fig. 341) which are the primary elements of the xylem lie at the 

 ends of the long axis of the bundle, and it is from these two points that the development 



