364 LYCOPODIALES 
From such a starting-point various lines of elaboration may be traced, 
open often to ready biological explanations: and these appear to .have 
run in some degree parallel in the ligulate and non-ligulate series. The 
steps which may be traced on a basis of comparison are as follows: 
First, the progressive sterilisation by abortion of sporangia increased the 
vegetative region: this led to more definite specialisation of the strobilus: 
in the more advanced forms the sporophylls are no longer nutritive, but 
only protective in function, so that the differentiation of the nutritive 
from the vegetative tract has become clearly marked. The vegetative shoot 
once distinct from the propagative strobilus was susceptible of various 
specialisation. In the dendroid fossils it attained large size, with secondary 
increase of its tissues, both stelar and extra-stelar, but still it maintained 
its radial symmetry. In the smaller forms, the straggling or climbing 
habit led not uncommonly to dorsiventral development, which occasionally 
extended to the more conservative strobilus itself. Such advances were 
accompanied by various elaboration of the vascular tissues, such as 
medullation, disintegration into separate strands, or even into meristeles. 
But these are all referable back in origin to the primitive monostele, just 
as the variations of external character are referable by comparison to the 
primitive strobilus. 
The sporangia all conform to one general fan-shaped type, with 
singular constancy of number and position relatively to the leaves. But 
the dimensions vary, and at least in Lycopodium there is a relation 
between the size of the sporangium and the definition of the strobilus: 
where the shoot is undifferentiated, as in Z. Se/ago, the sporangium is 
radially compressed: where the strobilus is clearly defined, and the 
vegetative region more specialised, as in LZ. clavatum or alpinum, it is 
radially elongated. The most extreme cases of this are found among the 
ligulate forms, as in *the dendroid fossils with their ample vegetative 
system. But, on the other hand, this relation is not constant, for 
the sporangia of Jsoe¢es are radially elongated, though there is no 
differentiation of the strobilus, while the sporangia of Se/aginella ate 
compressed; though the strobili are clearly defined. One of the most 
interesting points in these large sporangia is the partial sterilisation of 
their sporogenous tissues, probably to meet mechanical and _ nutritive 
requirements: sterile trabeculae are thus formed in the sporangia of 
Zsoetes, and in certain Lepzdostrobc. This leads towards a condition 
of septation, but in the Lycopods the step is never taken to complete 
partition of the sporangium. Finally, the heterosporous differentiation is 
probably a condition assumed after the character of the sporangium was 
already defined, and it has not greatly affected the general morphology 
of the shoots where it has occurred. 
In the eligulate series the embryo is simple and spindle-shaped. In 
L. Selago, which on other grounds is regarded as a primitive type, it 
grows directly and without complications into the seedling, with its green 
