= ELEMENTARY ORGANS. + 
g. In this modification of spiral vessels the tube is much lengthened, and the 
coil within it is either closed, that is, will not unroll, as in the ferns; or it is annu- 
lar, that is; broken into distinct rings, as in the garden balsam; or it is reticulated, 
that is, branching, the branches crossing so as to form a net-work. The office of 
all these ducts is the same, — that of conveying fluid. It is only in the spiral 
vessel that we find air. (Fig. 2, a, b.) . 
34. LATICIFEROUS TISSUE is so called, from /atez, the true 
nutritious sap, which it is destined to elaborate and convey. It 
consists of branched anastomosing (ava, to and fro, gtoua, a pas- 
sage) tubes, lying chiefly in the bark, and the under side of — 
leaves. (Fig. 2, c.) 
a. These tubes are very irregular in form, direction, and position. They 
expand and contract at intervals, cross and recross the other tissues, and, proceed- 
ing from the inner parts, ramify upon the outer surface, and upon the hairs, 
forming meshes of inconceivable fineness. Their average diameter is about 
rzo0 of aninch. They are largest in plants which have a milky latex, or juice. 
35. The EPIDERMIS, or skin, is a form of cellular tissue exter- 
nally enveloping the plant. It is found upon every part exposed 
to the air, except the stigma of the flower, and the spongioles 
of the roots. These it does not cover, nor is it found upon those 
parts which habitually live under water. And, where the bark 
of the stem is rugged with seams and furrows, this organ is not 
distinguishable. 
36. It consists of a tissue of flattened cells of various figures, 
filled with air. Usually there is but one layer of cells, but 
sometimes there are two or three, especially in tropical plants. 
The Oleander has four. Its office, in the economy of the plant, 
is, to check the evaporation of its moisture. . 
a. The delicate membrane, which may be easily stripped off from the leaf of 
the house-leek or the garden iris, is the epidermis. It is transparent, colorless, 
and, under the microscope, reveals its cellular structure. 
37. The epidermis does not entirely exclude the tissues be- 
neath it from the external air, but is perforated by certain aper- 
tures, called sromata (mouths), which open or close under the 
influence of the light. (Fig. 2.) 
38. Stomata are usually of an oval form, bounded by a pair 
of kidney-shaped cells, containing a green matter. Sometimes 
they are round, and bounded by several cells, Many 3 
varieties of form have been noticed 
