CHAP. II. 
LEAVES. 
121 
perfoliate. Frequently two opposite leaves grow together at 
the base, as in Caprifolium perfoliatum ; to this modification 
the latter term is often also applied, but that of connate is 
what more properly belongs to it. 
The anatomical structure of the leaf is this : — From the 
medullary sheath diverges a bundle of woody tissue, accom- 
panied by spiral vessels : this passes through the bark, and 
proceeds, at an angle more or less acute, to a determinate 
distance from the stem, branching off at intervals, and, by 
numerous ramifications, forming a kind of network. At the 
point of the stem whence the bundle of fibro-vascular tissue 
issues, the cellular tissue of the bark also diverges, accom- 
panying the fibro-vascular tissue, expanding with its ramifi- 
cations, and filling up their interstices. The tissue that 
proceeds from the medullary sheath, after having passed from 
the origin of the leaf to its extremity, doubles back upon 
itself, forming underneath the first a new layer of fibre, 
which, upon its return, converges just as the first layer 
diverged, at length combining into a single bundle, corre- 
sponding in bulk and position to that which first emerged, 
and finally discharging itself into the liber. If, therefore, a 
section of the leaf and stem be carefully made at a nodus, it 
will be found that the bundle of woody tissue which forms the 
frame-work of the leaf communicates above with the medullary 
sheath, and below with the liber. This is easily seen in the 
spring, when the leaves are young ; but is not so visible in 
the autumn, when their existence is drawing to a close. The 
double layer of fibrovascular tissue is also perceptible in a leaf 
which has laid during the winter in some damp ditch, where 
its cellular substance has decayed, so that the cohesion between 
the upper and lower layers is destroyed, and the latter can be 
easily separated. The curious Indian leaves which have the 
property of opening, upon slight violence, like the leg of 
a silk stocking, so that the hand may be thrust between their 
upper and lower surfaces, derive that singular separability 
from an imperfect union between the layer of excurrent 
and recurrent fibre. De Candolle remarks, that, when the 
fibres expand to form the limb of a leaf, they may (whether 
this phenomenon occurs at the extremity of a petiole, or at 
