24 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 latex, the true 

 nutritious sap, which it is destined to elaborate and convey. It 

 consists of branched anastomosing (*, to and fro, oro^a, a pas- 

 sage) tubes, lying chiefly in the bark, arid 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 

 r?W of an inch. 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 nigged 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 STOMATA (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 other 

 varieties of form have been noticed 



