HICK AND CASH : 
AFFINITIES OF LFPIDODENDROX. 
823 
As the stem advances in age this primary structure is supple- 
mented by the introduction of two additional masses of tissue, in the 
axile vascular cylinder and the cortex respectively. 
i In the axile vascular cylinder a ring of cambium appears 
between the primary xylem and the primary phloom, 
and gives rise to secondary vascular bundles, the xylem 
of which forms tlie ' vascular zone' of Williamson, 
ii. In tlie liypoderma of the cortex another cambhim ring 
makes its appearance, and gives rise to a sclerench}^- 
matous phelloderm, which forms tlie mechanical tissues 
of tlie cortex, the 'prosenchymatous tissues' of William- 
son, in a way analogous to that in wliich phelloderm is 
formed in existing gymnosperms and phanerogams. 
The secondary tissues thus produced, though tliey may turn out 
to be essential features of tlie genus Lepidodemlron, may be regarded 
as a part of the adaptations recj^uired for the aborescent liabit, and 
need not appear in otiier genera, whicli, with L(qndodendron , consti- 
tute a group of a higher order. In Dicotyledons we have genera and 
orders in which such secondary structures are constantly present, 
and others again in whicli tliey are constantly absent. Hence there 
is some justification for regarding the primary tissues of Lepidoden- 
dron as the only fundamental and characteristic ones, while the 
secondary tissues are due to adaptative requirements, and for taxo- 
nomic purposes must be relegated to the second line. 
In this connection it is worthy of note that while the cambium 
which arises in the vascularcylinder gives rise on the inside to tracheides 
only, with, of course, medullary rays, that of the liypoderma produces 
only sclerenchyma. Thus the production of vascular tissue is distinct, 
and separated in space from that of mechanical tissue, while in 
Dicotyledons, as is well-known, both forms of tissue arise from the 
same cambium. 
Whether in the case of the secondary vascular bundles, 
secondary phloem is produced in any quantity as well as secondary 
xylem is very doubtful. Similarly, the formation of phelloderm does 
not appear to have been accompanied by the formation of much 
