LYCOPSIDA 79 



primordium is only a few cells high, one conspicuous cell 

 on its adaxial surface undergoes a periclinal division to 

 produce a Hgule primordium (i). This soon gives rise to a 

 membranous ligule a few mm long which, for a time, is 

 much larger than the young sporophyll. Next, a velum initial 

 appears, from which is developed the velum (2) — a flange of 

 tissue which partly hides the sporangium in the mature 

 sporophyll, except for an oval opening called the 'foramen'. 

 The sporangium (3) arises as the result of pericHnal divisions 

 in a group of superficial cells near the base of the sporophyll, 

 on its adaxial side. The inner daughter cells are potentially 

 sporogenous, while the outer (peripheral) cells give rise to 

 the sporangium wall, three or four cells thick. Isoetes is 

 peculiar among living plants in that some of the potentially 

 sporogenous cells become organized into trabeculae of 

 sterile tissue which cross the sporangium in an irregular 

 manner. They subsequently become surrounded by a tapetal 

 layer which is continuous with the one derived from the 

 innermost layer of the sporangium wall. 



The general appearance of the base of a mature sporo- 

 phyll is indicated in Figs. 13I and 13H, representing a longi- 

 tudinal section and an adaxial surface view respectively. The 

 sporangia of the Isoetales are larger than those of any other 

 living plant and have a very high spore content indeed. The 

 sporophylls formed earHest in the year and which, therefore, 

 lie outermost on the apex of the corm are megasporangial 

 and contain several hundred megaspores. Those formed 



right angles to f); h, leaf base (adaxial view); i, l.s. leaf base; 

 J-L, development of leaf; m-o, development of male prothallus ; 

 p, antherozoid ; q, r, s, development of archegonium; t-x, develop- 

 ment of young sporophyte; y, megaspore with female prothallus 

 and young sporophyte 



(1, ligule; 2, velum; 3, sporangium) 



(a, after Hirmer; b, Magdefrau; c, d, Rauh and Falk; f, based 

 on Eames; J. k, Bower; m, n, o, Liebig; p, Dracinschi; q, r, s, 

 Campbell; t-y, La Motte) 



