66 THE FLOWERING PLANT. 
tory cavity. The continuity of this system of cavities and 
its connection with the exterior can be proved by a simple 
experiment. Take a large stalked leaf and immerse the 
lamina in water. Now apply the mouth to the cut end of 
the stalk and blow vigorously, when numerous small air-bubbles 
will escape from the surface of the part under water. The 
vascular bundles mostly run between the two regions of the 
mesophyll, and they are generally surrounded by parenchyma 
made up of small colourless cells. The ground-tissue may also 
contain sclerenchyma, especially in the leaves of grasses, and 
intercellular spaces containing secretions. ‘These spaces are 
formed by the breaking down of cells, and they contain 
ethereal oils, which are often odorous, ‘The fragrance of 
crushed myrtle leaves arises from this cause, and other examples 
of such secretory reservoirs are found in rue and in the 
common St. John’s wort (Hypericum perforatum). The leaves of 
the latter present from this cause numerous transparent spots, 
which appear like perforations, whence the specific name. 
The vascular bundles of the leaf, in dicotyledons as well as 
monocotyledons, are devoid of cambium. The wood is above, 
and its tubular elements are always tracheides, never vessels. 
The bast presents the same structure as in the stem, but its 
elements are small and difficult to make out. It underlies the 
wood. The respective upper and lower positions of wood and 
bast, which also obtain in the petiole, may be understood by 
turning up the leaf into a vertical position, when what were 
upper and lower now become inner and outer. The mature 
leaf differs from root and stem in possessing ”o growing-potnt, 
and since cambium is absent, its growth is limited. ‘This accords 
with its comparatively transitory character. It is interesting to 
note that when the petiole is used for climbing, it often thickens 
and persists. 
It must not be imagined that all leaves possess the bifacial 
structure described in the preceding paragraph. When the 
general form is radially symmetrical, so also is the structure. 
Such leaves are said to be centric, like those of the stonecrop. 
Here there is no palisade parenchyma, but this may also be 
absent in flat leaves, e.g., those of grasses. On the other hand, 
vertical leaves have palisade parenchyma on doth sides, which 
also are equally rich in stomata. 
PHYSIOLOGY. 
As in the stem (p. 41), so also in the foliage leaf support is 
afforded by the firmer part of the vascular bundles, by scleren- 
