Ly cop o diales. 163 
Spencerites were more or less spherical, and as shown in the above 
discussion this spherical form of sporangium was probably primitive 
for the order. Professor Bower, who was the first accurately to 
compare the sporangia of Lepidostrobus and of Isoetes, and who 
favours the view of the reduction of the latter from the Lepidoden- 
draceae, has pointed out the resemblance between the trabeculae 
that render the sporangium of Isoetes incompletely septate and 
certain sterile plates of tissue found in the sporangia of Lepido¬ 
strobus Brownii (3). It is true that he was unable to prove a 
connection between the sterile plates and the upper sporangial wall 
of the fossil, and as the sterile bands extend upwards from the sub- 
archesporial pad they may only be out-growths from it, and not 
formed, as they are in Isoetes, by the sterilization of potential 
sporogenous tissue. The point is not quite proved, but the available 
data incline one to the view that the structures are strictly com¬ 
parable in the two genera, though they may, even so, have arisen 
independently in response to a demand for better nutrition of the 
spores, consequent on the radial expansion of the sporangium. 
The velum of Isoetes offers at best a poor analogy with the 
integument of Lepidocarpon, for in Isoetes the velum, when present, 
appears to arise distally, whereas the integument of Lepidocarpon 
originates chiefly from the sides of the sporophyll. The velum is 
moreover very inconstant in Isoetes, being present poorly developed, 
or absent in closely allied species. A character cf minor importance, 
but suggestive when combined with so many other indications of 
affinity, is the persistence of the leaf-bases on the stem after the 
lamina has withered. 
The arguments against the reduction of Isoetes from the 
Lepidodendraceae include three of considerable importance. The 
first of these is that the central cylinder of the stem of Isoetes has 
been described by Professor Farmer as having no truly cauline part 
(6), whereas the cauline part of the stele is well-developed in the 
Lepidodendraceae. Dr. Scott and Mr. Hill, however, after a 
thorough investigation of the anatomy of Isoetes hystrix concluded 
that this was not so in the mature stele of that species (18). They 
point out that it is arbitrary to regard the stele as entirely built up 
of leaf-traces when it is manifestly impossible to refer all its con¬ 
stituent elements to the particular leaf-trace to which they belong, 
and they quote Professor Farmer’s admission that the distinction 
lies rather in the mind of the investigator than in the stele itself. 
It is probable that at least in the terrestrial species with their better 
