Lycopodiaceous Cones from the Lower Coal Measures. 385 
3. That their cones were originally similar and have reached their 
present state after passing through similar series of stages ; this is, I believe, 
expressed by Osborn’s term Rectigradation. 
The fact that the cones of Lycopodium and Phylloglossum agree very 
closely with this type, except in the absence of the ligule, I think lessens 
very greatly the probability of the second explanation being correct. The 
assumption here is that, as the ligule is an obviously important organ in the 
heterosporous Lycopods, it is rather unlikely that the homosporous forms 
should have reached the same result in its absence, starting from different 
material. 
We are, therefore, left with two theories (the first and third above) to 
account for the resemblances between these cones ; these theories do not 
differ very much, and I propose to take the first as a working hypothesis and 
discuss the third later. 
It seems pretty safe to say that Lycopodium , Selaginella , Miadesmia , 
and Bothrodendron parted company a considerable time ago, and hence, 
assuming the first theory to be true, we may suppose that the cone of 
Bothrodendron mundum affords a pretty accurate representation, morpho- 
logically, of the primitive Lycopod cone, which would of course be homo- 
sporous. 
Assuming this, let us investigate what would be the effect of the 
development of an arboreal habit by the Lepidodendraceae. 
There is no doubt that it would lead to an increase in the number 
of spores required. This increase might be supplied in two ways. 
1. By increasing the number of sporophylls, either by increasing the 
number of cones or adding to their length. 
2. By increasing the size of the sporangium. 
Increase in the number of component parts is a very clumsy method of 
achieving the desired result, for a cone with small sporangia has a much 
smaller ratio of sporogenous to sterile tissue than has a cone with larger 
sporangia; compare, for example, Bothrodendron mundum vti\hLepidostrobus. 
Similar considerations show that the most efficient method of adding to 
the volume of a sporangium is by increasing its radial extension, for 
a greater height would not only increase the length of axis per sporophyll, 
but also the size of the lamina required for adequate protection of the spores. 
Hence, simple geometrical considerations show that it is likely that the 
adoption of an arboreal habit would lead to a radial extension of the spor- 
angium, assuming that such habit necessitated increased spore production. 
If we suppose the stalk of the Bothrodendron mundum sporophyll 
to be elongated, we get an arrangement by which the radial extension 
of the sporangium is increased. 
If we imagine this to have taken place, we are left with an arrangement 
recalling that of Spencerites . 
