16 BOTANY. 
4. Leaves and the Appendages. 
A leaf is a symmetrical lateral expansion of the bark, and is intimately 
connected with the internal part of the internal axis. Leaves, at first. are 
mere projections of cellular tissue, closely united to each other ; subsequently 
they enlarge with the addition of vascular tissue, and finally assume a 
permanent form and position along the axis. Whenever a leaf-bud is 
formed, a leaf is also; this, if not entirely developed, is at least rudimentary. 
Two ae edinendens of the leaf have been observed aie = as the 
medium of existence is air or water. 
In aerial leaves we have a skeletal vascular tissue in the form of veins, 
ribs, or nerves, the interspaces filled up by cellular tissue in the shape of 
parenchyma, the whole invested by epidermis. ‘The vascular system is 
continuous with that of the stem; the vessels from the interior of the stem 
spread out on the surface, the more external in the former appearing on the 
inferior face of the latter. This is well illustrated by the fact, that in the 
upper part of the leaf we find spiral vessels and woody fibre, in the lower 
there are laticiferous vessels and fibres like those of liber. The vascular 
system is distributed in the form of simple or branching veins. 
The epidermis generally differs on the two sides of the leaf. Thus, it is 
on the under face that the stomata are found in largest quantity, sometimes 
exclusively ; hairs also are of much more frequent occurrence. In leaves 
floating on the surface of water the stomata are superior. The parenchyma 
of the leaf is that cellular tissue fillmg up the interspaces of the vascular 
fibre. Other names are diachyma and mesophyllum. This parenchyma 
exhibits two series of cells, different in form and arrangement. 
Submerged leaves, or those developed under water, exhibit many points 
of difference from aerial leaves. There is here no fibro-vascular system, 
but merely an aggregation of cells, which sometimes simulate veins. There 
is no true epidermis, nor are there any stomata. Sometimes there is only 
a net-work of filamentoid cells, the interspaces not filled with parenchyma. 
Such leaves are called fenestrate. 
The ordinary leaf in its fullest form consists of an expanded flat portion, 
called the blade, or laminar merithal ; of a narrower portion, called stalk, 
petiole, or petiolary merithal, which is continuous with.the midrib ; and 
sometimes of a portion at the base of the petiole, forming a sheath or vagina ; 
the latter, again, may be developed in the form of small leaves, called 
stipules. When a leaf has a distinct stalk, it is said to be petiolate ; when 
there is no stalk, it is called sessile. When sessile leaves embrace the stem 
they are said to be amplexicaul. The portion of the leaves next to the stem 
is the base, the opposite extremity is the aper. The surfaces of the leaves 
are called the pagine ; the edges or margin, the circumscription. The usual 
position of the leaf is horizontal; sometimes it is vertical, or else between the 
two positions. The upper angle, formed by the petiole with the stem, is the 
axilla ; anything arising from that part is azillary. The petiole is some- 
times articulated with the stem, leaying a scar on the latter when it falls; 
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