44 



VASCULAR CRYPTOGAMS 



the development and lignification of the tracheides advance centripe- 

 tally. The external layer of phloem is itself surrounded by two or 

 three parenchymatous layers, constituting a bundle-sheath, belonging 

 to the fundamental tissue, but within the large air-cavity. The mode 

 of apical growth varies in the different species. In some the apex of 

 the stem is occupied, as in Isoetes, by a group of equivalent meris- 

 matic cells ; while in others there are" two co-ordinate apical cells side 



by side, or a single one, 

 which may be two-sided or 

 three-sided. 



The leaves are simple 

 and unbranched, and are 

 traversed by a single 'vas- 

 cular' bundle. They are 

 always of small size, re- 

 sembling those of Lycopo- 

 dium, awl-shaped and acu- 

 minate, or ending in a 

 delicate awn, and usually 

 with a cordate base. The 

 greater number of species 

 are heterophyllous, the 

 sterile leaves having two 

 different forms ; those on 

 the ventral or shaded side 

 of the obliquely ascending 

 stem are larger than those 

 on the dorsal side exposed 

 to the light. They are 

 always in four rows, one 

 dorsal and one ventral leaf 

 forming a pair. On the 

 upper side of the leaf near 

 its base is; the peculiar 

 structure known as the ligule, from the presence of which the class has 

 sometimes been called ' Ligulatse.' The fertile leaves are uniform in size, 

 and differ somewhat in shape from the sterile, forming a compact square 

 terminal spike. The sporange springs from the upper surface below the 

 ligule. In some species the epiderm is alike on the two sides of the leaf; 

 in others it differs. The epidermal cells contain chlorophyll, as is the case 

 in ferns, and frequently have beautifully serpentine lateral walls ; in some 

 species they are so greatly, thickened that theL cell-cavity disappears altO' 



Fig. 25. — S . imtqualifolia ; transverse section of stem 

 ( X 150). (After Sachs.) 



