THE CLUB-MOSSES 167 



is that the stem grows indefinitely in thickness 

 by means of cambium, a rare occurrence among 

 living Cryptogams. A considerable amount of 

 secondary wood and bast is added by the cam- 

 bium, but the cortex is often the part chiefly 

 increased. The whole process is peculiar and dif- 

 fers from the normal secondary growth of a 

 Dicotyledon or Gymnosperm. It is interesting 

 to note that growth in thickness also goes on in 

 the base of the stem of Selaginella spinosa. The 

 leaves of Isoetes have the single vascular strand 

 characteristic of the class. As in Selaginella, the 

 ligule is a constant feature. 



The roots are remarkable in so far as their 

 structure somewhat resembles that of a leaf, 

 the vascular system consisting of a single strand 

 of wood and bast, whereas in typical roots there 

 are two or more strands of each. This peculiarity 

 is also frequent in the roots of Selaginella. 



The sporangia of Isoetes are unusually large, 

 often quite a quarter of an inch long. Each is 

 seated in a hollow on the upper side of the fertile 

 leaf, between its base and the ligule. There is 

 often a membrane, like the integument of an 

 ovule, which wholly or partially envelops 

 the sporangium. It will be noticed that in 

 Isoetes the sporangium is entirely a product of 

 the leaf, and occupies a considerable part of the 

 leaf-base. It is rather a complex structure, for 



