144 STRUCTURE OF AERIAL LEAVES. 
therefore corresponds in its structure to the wood, and the lower 
to the liber ; hence the former is composed of spiral and pitted 
vessels in perennial plants, and of spiral and annular or some 
other vessels in herbaceous plants (fig. 276, t, t), and also in all 
cases, of wood-cells, f, besides the above-named vessels ; while 
the latter consists essentially of liber-cells, /, 1, and sieve-tubes. 
The ramifications of the fibro-vascular elements in the lamina of 
the leaf form the veins or nerves, and will be described presently 
under the head of venation (see page 157). The number and 
size of the elements of the bundle diminish however as they 
continue to ramify, so that the ultimate degrees of ramification 
commonly consist of spiral vessels alone. The two layers of the 
fibro-vascular system are usually readily seen in what are called 
skeleton leaves. Thus the leaves lying in a damp ditch in the 
winter will afford us good illustrations of these, and those which 
have been artificially prepared by maceration for a sufficient 
time in acidulated water, or in other ways. 
Rie ii. 
St 
{ 
Fig. 278. 
Fig. 277. Vertical section of a leaf of the Melon, highly magnified. es. Epi- 
dermal tissue of the upper surface furnished with hairs, p, and stomata, 
st. ei. Epidermal tissue of the lower surface, with the hairs arising from 
it. ps. Three layers of parenchymatous cells below the epidermis of the 
upper surface. pi. Parenchymatous cells below the epidermal tissue of 
the lower surface. jv, fv. Fibro-vascular tissue forming the veins. m, m, 
Cavities connected with the stomata. J, 7. Cavities between the loose 
spongiform parenchyma,——Fig. 278. Vertical section of a leaf of the 
White Lily highly magnified, showing the epidermis of both the upper 
and lower surfaces, with the intervening parenchyma, 
2. Parenchyma or Mesophyll.—By this we understand the 
parenchymatous tissue which is situated between the epidermis 
of the upper and lower surfaces of the lamina (fig. 277, ps, pi), 
and which surrounds the ramification of the fibro-vascular 
system or veins, fv, fv. The parenchymatous tissue which is 
