318 LYCOPODIALES 
In J/soefes the position of the large sporangium, between the ligule 
and the axis, corresponds to that in Seéagzzed/a, 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, Cc, 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 
Fic. 165. 
Tsoetes lacustris, L. A=radial section through base of sporophyll with ligule (2), 
velum (v), and sporangium, in which the archesporium is shaded. B=a similar section 
of an older sporangium. C=part ofan older microsporangium, showing the potential 
archesporium differentiated into trabeculae (¢~), and sporogenous tissue (sf), while the 
tapetum (¢) is clearly defined J=an older stage with spore-mother-cells separated, and 
tapetum shaded covering the trabeculae. A, Bx2d0. C, Dx100. 
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, 
