HICK AND CASH : AFFINITIES OF LEPIDODENDRON. 
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4. Lastly, the whole of the preceding tissues are enclosed by 
the external cortex [e). This " consists of strongly defined thick- 
walled cells which diminish from within outwards." " The smaller 
peripheral cells gradually pass over into crescentic clusters of radially 
disposed prosenchymatous cells This prosenchymatous tissue, 
according to Williamson, arises first in that part of the outer cortex 
which underlies the leaves and subsequently becomes a thick con- 
tinuous zone, growing in breadth as the stem increases in age, by 
means of a cambium layer situated at its outer periphery. 
From this description of the external cortex it is evident that we 
have here two distinct tissues, which it will be advantageous to keep 
separate. The strongly defined thick- walled cells of the inner zone {e) 
represents a part of the primary cortex, and to it, in strictness, the 
term external cortex should be confined. The prosenchymatous 
cells id) are absent from the primary cortex of young twigs. They form 
indeed a ^econdanj tissue, arising from a secondary meristem, and may 
be considered as a kind of phelloderm superadded to the primary 
tissues. 
5. At the edge of the transverse section the external cortex 
runs out into the bases of the leaves {g). 
6. In addition to the tissues already enumerated and described, 
and which may be regarded as fundamental and characteristic, the 
stem of Lepidodendron selaginoides when fully developed has a 
mass of secondary vascular tissue. This is the tissue that Williamson 
speaks of indifferently as ' the exogenous zone,' ' the vascular zone,' 
or ' the ligneous zone.' When present this zone of tissue is inter- 
calated between the ' vascular axis ' and the * innermost cortex.' It 
consists of thin wedge-shaped masses of elongated scalariform trach- 
eides, separated by medullary rays, and though often unilateral 
at first, forms at length a complete zone round the ' vascular axis.' 
The elements of these wedges increase in size from within outwards 
until a certain maximum is attained, after which no further increase 
occurs. By the action of a cambium ring the zone grows exogenously — 
i.e., by additions to the vascular wedges and the medullary rays at 
the outer periphery — and in this way an increase in the diameter of 
the stem is brought about. 
