350 Bower . — On the Structure of the Axis 
weight in tracing relationship. The very great variety in 
detail of the cortex in the living species of Lycopodium would 
suffice to shake confidence in arguments upon such a basis, 
and to suggest that these details are determined by a rela- 
tively direct adaptation. It is, however, worthy of notice 
how similar the modes of variation have been in the plants of 
the past to those of their living relatives. The same general 
type, even of the cortical tissues, runs through all these 
Lycopodinous forms, though that type is certainly less dis- 
tinctive than are the leading characters of the vascular 
tissues. After studying such fluctuating characters as these 
of the cortex, one returns with renewed confidence to the 
vascular system. Whatever its differences of detail, the 
Lycopodinous vascular system is a distinctive one ; a com- 
parison of the stele of Lycopodium Selago, Selaginella spinosa , 
Lepidodendron selaginoides , Lepidostrobus Brownii , of Psilo- 
tum triquetrum and Tmesipteris truncata , shows a unity of 
plan which cannot be missed ; occurring, as this does, among 
plants which have always been classed together, the unity of 
general vascular plan must be accepted as strengthening their 
relationship. Perhaps the most interesting case of all is the 
similarity between Lepidostrobus Brownii and the Psilotaceae, 
depending upon the points already noted above. It is to be 
remarked that while Brown’s cone compares with Psilotum as 
regards its endodermis, it approaches Tmesipteris in the soft 
central parenchyma ; again, the similarity between the axis of 
Brown’s cone and that of Selaginella spinosa is interesting, 
though not so close in detail as the above ; for, in addition to 
the details of the vascular tissue, there is similarity in the 
existence of the trabeculae, though not in their distribution. 
Again, many of the Lepidostrobi are proved to be, like Sela- 
ginella, heterosporous, though this proof is not present for 
Brown’s cone, which only bears microsporangia ; it is, how- 
ever, an obviously incomplete cone, so that the absence of 
megaspores does not prove it homosporous. Lastly, like 
Selaginella , certain Lepidodendra have been shown to possess 
a ligule ; I have not, it is true, been able to see any ligule in 
