THE LEAF. 
69 
much reticulated in addition, the surface is described 
as being reticulate. 
The epidermal cells are further characterized by the 
development of stomata and water-pores, the origin 
and function of which have already been described. 
The parenchyma of leaves, known as mesopliyll, may 
be of two kinds: one in which the cells are nearly 
isodiametric, or branching, and between which are 
large intercellular spaces; and another, consisting of 
elongated cells, which are compactly arranged, and 
known as the palisade cells. The latter usually occur 
beneath the upper epidermis, as in coca, digitalis and 
hyoscyamus ; or they may occur upon the lower surface 
as well, as in senna; or they may constitute the entire 
parenchyma of the leaf, as in eucalyptus. Oil-secre- 
tion reservoirs are sometimes found in the parenchyma, 
as in pilocarpus and eucalyptus, when the leaf is said 
to be pellucid-punctate. 
The fibrovascular tissue of the leaf consists of xylem 
and phloem, surrounding which is a layer of cells cor- 
responding to the endodermis or bundle-sheath of the 
young stem. This tissue is found in the middle of the 
veins, the phloem being situated on the side toward 
the upper surface of the leaf, and the xylem toward 
the under surface. Collenchyma is frequently found 
under the epidermis of the veins of the leaves, as in 
Mentha piperita, senna and stramonium. 
(ff) PHYLLOTAXY Oil PHYLLOTAXIS. 
Phyllotaxy, or phyllotaxis, is the study of the dis- 
tribution of leaves upon the stem, and of the laws 
which govern it. If we examine germinating plants 
of the beech, the elm, or the oak, we observe that, while 
the seed-leaves are opposite to each other, the subse- 
quent leaves are arranged according to a different order 
