318 LYCOPODIALES 



In Isoetes the position of the large sporangium, between the ligule 

 and the axis, corresponds to that in Selagznella, though it is here more 

 definitely inserted on the leaf-base, and is sunk in a deep depression of 

 its upper surface (Fig. 155 B, c, D) ; but these differences of detail do 

 not obscure the essential unity of the plan in the two genera. Instead 

 of being a body more or less flattened between the sporophyll and the 

 axis, as in Lycopodium and Selaginella, the sporangium is here extended 

 radially outwards from the axis into a broad cake-like body. It may 

 best be regarded as a result of such variation of dimensions as has been 



FIG. 165. 



Isoetes lacustrh; L. A radial section through base of sporophyll with ligule (/), 

 velum (f), and sporangium, in which the archesporium is shaded. B &. similar section 

 of an older sporangium. C = part of an older microsporangium, showing the potential 

 archesporium differentiated into trabeculae (t>-\ and sporogenous tissue (sfi), while the 

 tapetum (t) is clearly defined /-> = an older stage with spore-mother-cells separated, and 

 tapetum shaded covering the trabeculae. A, Z>'X 200. C, D x 100. 



seen in minor degree within the genus Lycopodium, but here carried to 

 greater lengths. The developmental details harmonise readily with this 

 view. The microsporangium is naturally a better basis for comparison 

 with the homosporous Lycopods than the megasporangium, and it will 

 therefore be taken first The mature structure of a microsporangium is 

 shown in Fig. 155 D, which indicates how the very large internal space 

 is traversed by the sterile trabeculae : these extend, with many irregularities 

 of branching and wing-like expansions, which are not shown in the figure, 

 from the sub-archesporial tissue to the covering wall. The type of the 

 megasporangium is the same, though the trabeculae are here fewer in 

 number but more massive, so that the proportion of sterile tissue to the 

 fertile is much larger in the megasporangium. As the development shows, 



