98 
ORGANOGRAPHY. 
BOOK I. 
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 imderneath 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 : they can then 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 separa- 
bility 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 
the point of separation from the stem) do so after two different 
systems : they may either constantly preserve the same plane, 
when common flat leaves are formed ; or they may expand in 
any direction, when cylindrical or swollen or triangidar leaves 
are the result. (Organogr, p. 270.) 
The cellular tissue of which the rest of the leaf is composed 
is parenchyma, which Link then calls diachyma^ or that im- 
mediately beneath the two surfaces cortex, and the intermediate 
substance diploe. De Candolle calls these two, taken to- 
gether, the rnesophyllum. The wliole is protected, in leaves 
exposed to air, by a coating of cuticle, furnished with stoma- 
