324 LYCOPODIALES 



the flange of insertion an internal ridge of sterile tissue extends upwards 

 into the sporangium, just as in Isoetes (r. 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 E). 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 Isoetes, to which they show a 

 striking similarity. It seems probable that they are truly comparable to 

 the trabeculae of Isoetes; 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 L. Brownii 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 plant of Isoetes are such 

 as can hardly have been the result of parallel development : they strongly 

 support the view expressed above, that the plant of Isoetes is like a 

 stunted Lepidodendron. 



On the other hand, Brown's cone shows only microsporangia, while 

 Isoetes^ like certain other Lepidostrobi, 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 Z. 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 Lepidostrobi 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 Isoetes. 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 



