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 L. Selago, the sporangium is 

 radially compressed : where the strobilus is clearly defined, and the 

 vegetative region more specialised, as in L. 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 Isoetes are radially elongated, though there is no 

 differentiation of the strobilus, while the sporangia of Selaginella are 

 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 

 Jsoetes, and in certain Lepidostrobi. 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 

 /.. St'/ago, which on other grounds is regarded as a primitive type, it 

 grows directly and without complications into the seedling, with its green 



