THE FORMATION OF WOOD IN PLANTS. 423 
tissues to assume their typical structures. To which admission, however, it must be 
added that since this typical structure assumed, though imperfectly assumed, by the hot- 
house plant, is itself interpretable as the inherited effect of external mechanical actions 
on its ancestors, we may still consider the current set up by the assumption of the typical 
structure to be indirectly due to such actions. 
Interesting evidence of another order here demands notice. In the course of experi 
ments on the absorption of dyes by leaves, it happened that in making sections parallel 
to the plane of a leaf, with the view of separating its middle layer containing the vessels, 
I came upon some structures that were new to me. These structures, where they are 
present, form the terminations of the vascular system. They are masses of irregular 
and imperfectly united fibrous cells, such as those out of which vessels are developed ; 
and they are sometimes slender, sometimes bulky—usually, however, being more or less 
elub-shaped. In transverse sections of leaves their distinctive characters are not shown ; 
they are taken for the smaller veins. It is only by carefully slicing away the surface of a 
leaf until we come down to that part which contains them, that we get any idea of their 
nature. Fig.1 (Plate LIV.) represents a specimen taken from a leaf of Euphorbia neriifolia. 
Occupying one of the interspaces of the ultimate venous network, it consists of a spirally 
lined duct or set of ducts, which connects with the neighbouring vein a cluster of half- 
reticulated, half-scalariform cells. These cells have projections, many of them tapering, 
that insert themselves into the adjacent intercellular spaces, thus producing an extensive 
surface of contact between the organ and the imbedding tissues. A further trait is, 
that the ensheathing prosenchyma is either but little developed or wholly absent; and 
consequently this expanded vascular structure, especially at its end, comes immediately 
in contact with the tissues concerned in assimilation. The leaf of Euphorbia -neriifolia 
is a very fleshy one; and in it these organs are distributed through a compact, though 
watery, cellular mass. But in any leaf of the ordinary type which possesses them, they 
lie in the network parenchyma composing its lower layer; and wherever they occur in 
this layer its cells unite to enclose them. This arrangement is shown in fig. 2, repre- 
senting a sample from the Caoutchouc-leaf, as seen with the upper part of its envelope 
removed; and it is shown still more clearly in a sample from the leaf of Panax Lessonti, 
fg. 3. Figures 4 and 5 represent, without their sheaths, other such organs from the 
leaves of Panaw Lessonii and Clusia flava. Some relation seems to exist between their 
forms and the thicknesses of the layers in which they lie. Certain very thick leaves, such 
as those of Olusia flava, have them less abundantly distributed than is usual, but mae 
massive, Where the parenchyma is developed not to so great an extreme, though still 
largely, as in the leaves of Holly, Aucuba, Camellia, they are not so bulky; and In 
nner leaves, like those of Privet, Elder, &c., they become longer and less conspicuously 
- d. Some adaptations to their respective p tions seem implied by these 
modifications; and we may naturally expect that in many thin leaves these free ends, 
ming still narrower, lose the distinctive and suggestive characters possessed by those 
in the diagrams. Relations of this kind are not regular, however. In veriona 
Senera, members of which I have examined, as Rhus, Viburnum, Griselinia, 
a, Botryodendron, Pereskia, the variations in the bulk and form of these re 
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