324 LYCOPODIALES 
the flange ‘of insertion an internal ridge of sterile tissue extends upwards 
into the sporangium, just as in Jsoetes (7. Fig. 169 B,D), while from it 
sterile processes project further upwards, extending far into the cavity, 
and traversing the mass of the spores (Fig. 169 £). In the mature 
sporangium they stop short of the upper sporangial wall, but in the young 
state—as seen in the arrested sporangia towards the apex of the cone— 
they may extend completely across the cavity: in position and in number 
they are irregular, as are the trabeculae of Jsoe¢es, to which they show a 
striking similarity. It seems probable that they are truly comparable to 
the trabeculae of Zscetes; but, on the other hand, it is possible that they 
may correspond rather to those irregular upgrowths from the sub-arche- 
sporial tissue mentioned as occurring in some of the larger sporangia of 
Lycopodium. The large cavities of the sporangia are filled with small 
spores, arranged in tetrads, and it is probable that the trabeculae were 
of importance in the nourishment of the large sporogenous mass, as 
also mechanically. The wall of the sporangium in Z. Browniz consists 
of an outer layer of indurated prismatic cells, supported by four or more 
layers of thin-walled cells (Fig. 169 A). It is impossible to miss the 
general similarity of this large sporangium to the microsporangium of 
Isoetes: the size, the position, the outline, and the presence of trabeculae 
all point to the close correspondence: a ligule has, it is true, not been 
noted in the fossil; but as only a few sections have been available, and 
as the ligule in other Lepidodendrons is only small, it would be rash to 
lay any great stress upon this negative observation. The points of 
similarity of this remarkable fossil to the fertile plantgzof /soe/es are such 
as can hardly have been the result of parallel development: they strongly 
support the view expressed above, that the plant of Jsoe¢es is like a 
stunted Lepzdodendron. 
On the other hand,*Brown’s cone shows only microsporangia, while 
Isoetes, like certain other Zefidostrobi, is heterosporous. But the specimen 
itself was incomplete: only the upper part of the cone is represented, and 
it is now known that in other species the apical region bore microspor- 
angia, while the lower bore megasporangia, as in L. Veltheimianus (Fig. 
170): it is quite possible that the lower portion, which is missing from 
Brown’s cone, bore megasporangia; but on this point there is no positive 
evidence. 
In other Lepzdostrobi the general form of the sporangium is the same 
as that above described: there is great radial extension, while in a number 
of cases a ligule has been found at the distal end, thus corresponding in 
position to that in Jsoe¢es. The sporophylls are liable to peltate expansion 
at the apex: they are then so disposed that the downward-turned lips of 
the upper sporophylls are enveloped by the upturned lips of the lower, 
thus giving very complete protection to the sporangia. This may be held 
to be a secondary adaptation of their form, comparable to that seen in 
some of the more specialised cones of Lycopodium belonging to the 
