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 trachei'des, 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 no growing-point, 

 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 both 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- 



