348 Bower . — On the Structure of the Axis 
walled. Certain species of Selaginella correspond to this last 
type, in respect of the structure of the cortex. .S'. Martensii 
and 5. Willdenovii show a peripheral, firm and woody band, 
which becomes more delicate on passing inwards, till finally 
the lax trabeculae round the bundle are reached. vS. spinosa 
is also of the same type, but in this species the sclerosis is 
limited to the epidermis \ while the inner part of the cortex 
is not only thin-walled, but contains large lacunae, which open 
into the air-space round the bundle. 
Finally, the cortex of the Psilotaceae shows an absence of 
large intercellular spaces ; in the old stem of Tmesipteris it is 
a dense band of sclerotic tissue, while the layers immediately 
outside the sheath show that peculiar brown thickening of the 
walls already described by others 1 2 . In Psilotum , the epi- 
dermis, with its thick outer wall, is succeeded by three or four 
layers of thin-walled chlorophyll-parenchyma, within which 
is a sclerotic band, and this is again succeeded by a thinner- 
walled cortex and endodermis. 
From these descriptions of the cortex in living Lycopods 
and Psilotaceae, and in Lepidostrobus Brownii , and various 
stems of Lepidodendron , it is clear that these plants show 
considerable variety of detail in the nature of this tissue ; 
a special point of interest for us is that those different types of 
structure which obtained in the fossil forms can be matched 
by similar characters In living forms of close alliance. Thus 
the curious intertwined filamentous middle cortex of Lep. 
fuliginosinn finds its counterpart in the middle cortex of 
L. Selago, though in a less pronounced manner. The case 
of Lep. Harcourtii , where the innermost firm band appears 
not distinctly differentiated, is matched by L. nummulari - 
folium , or (exclusive of the large air-space round the steles) by 
Selaginella Martensii or Willdenovii . But the most inter- 
esting for comparison are those with large air-spaces ; the 
case of Lep. selaginoides , with the large lacunae and con- 
necting trabeculae in the middle cortex, resembles what is 
1 De Bary, Comp. Anat., p. 430. 
2 Bertrand, loc. cit. p. 482, and Figs. 207-212. 
