SELA GIXEL LA CE.E 



51 



the fovea, and separated from it by a ridge called the saddle, is a smaller 

 depression, thefoveola, the lower margin of which forms a lip-like struc- 

 ture, the labium, and from its base rises a narrow membranous acuminate 

 structure, the ligule, with a cordate base, and usually projecting above 

 the foveola. The sheath passes above into the lamina, which is narrow 

 and thick, almost cylindrical, but flattened in front, contains chlorophyll, 

 and is traversed by four wide longitudinal air-cavities, segmented by 

 transverse septa. A rosette of these fertile leaves or sporophyUs is pro- 

 duced annually, but between these whorls are alternate whorls of phyl- 

 lades, or imperfec; leaves, consisting, in the submerged species, of only 

 a small lamina with no sheath, while in the terrestrial species they are 

 reduced to mere scales. Stomates occur in the paludose and terrestrial, 

 but not in the submerged species. Scattered spiral tracheides are found 

 in the parenchymatous base of the leaf. The fundamental tissue, which 



Fig. 33. — Development of microsporange of /. laciestris. t, tapetal cells ; Tr, trabecules : the 

 archespore and sporogenous cells derived from it are shaded. (After Goebel, magnified.) 



is not separated from the single ' vascular ' bundle by a bundle-sheath, 

 has a strong tendency to become sclerenchymatous, especially beneath 

 the epiderm and in the sheath. The very simple bundle which occurs 

 in each leaf is stated by Russow to be collateral, the xylem and phloem 

 lying side by side. 



The roots spring from the furrows of the stem, and resemble, in 

 structure and mode of branching, those of Selaginella. There is no 

 rhizophore. 



The sporanges of Isoetes do not make their appearance until the 

 third year after germination. Each sporophyll bears only a single 

 sporange, which is undoubtedly a product of the leaf, and is situated 

 below the ligule in the fovea, to which it is attached by a narrow base. 

 The outer leaves of the fertile rosette produce megasporanges only, the 

 inner leaves microsporanges only. Both kinds of sporange originate 



from a group of cells at the base of the leaf The archespore is derived 



E 2 



