336 Groom . — On Thismia Aseroe {Bee cart) 
be correct, then the exocortex , inasmuch as it is the outermost 
layer which soonest ceases undergoing periclinal divisions 
and becomes earliest sharply marked off from the cells 
produced by the calyptrogen, is the original piliferous layer 
(epiblema). Undoubtedly the cells which produce it also 
give rise to the layers as far as the endodermis, but it was 
impossible to see whether the initials of this layer and the 
cortex are absolutely distinct from those forming the sheath. 
The limiting layer then corresponds with the exodermis . The 
rest of the cortex is equivalent to the remaining cortical layers 
of an ordinary root. The section figured suggests that the 
plerome has initials of its own (Fig. 4). The structure of the 
central cylinder favours the view that this vegetative axis is 
a root. There is in other saprophytic roots, and in Burman- 
niaceae in particular, a tendency for the xylem-bundles to be 
developed towards the centre of the cylinder; and there is 
the further fact that the spiral and annular vessels tend to lie 
outside the scalariform-reticulate tracheides. 
But another interpretation is possible, namely, that the 
vegetative axis is a rhizome in which the leaves are con- 
genitally fused with the stem. This view may be illustrated 
by some examples which offer stages towards this process 
of fusion. On the tuberous rhizome of Epipogum nutans , the 
leaves are well developed but without vascular bundles, and 
their inner (upper) faces are so closely adherent to the axis 
that they have in sections to be separated by chemical means 1 . 
In addition the lower (outer) surface of the leaves is provided 
with numerous absorbing hairs. If we imagine this state in 
the ancestor of Thismia , and that gradually the leaves have 
become congenitally fused with the axis, the present structure 
would result. In this case the sheath represents the leaf- 
tissue, the exocortex represents the epidermis of the axis, the 
tissue within corresponds to the cortex and stele of the stem. 
This view is supported by the fact stated by Johow 2 , that in 
1 Percy Groom, loc. cit. 
2 Johow, Die Chlorophyllfreien Humuspflanzen, &c. Prings. Jahrb. xx, 
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