24 nLEMKNTARy ORGANS 



g. In this modification of spiral vessels the tube is much lengthened, md tlia 

 soil within it is either closed, that is, will not unroll, as in the ferns-, or it 's «/?««- 

 Sir, that is, broken into distinct rings, as in the garden balsam ; or it is re iculated, 

 that is, bi-anching, the branches crossing so as to form a net-work. The )fSce of 

 all these ducts is the same, — that of convej-ing fluid. It is only in thi spiral 

 Tcssel that we find air. (Fig. 2, a, b.) 



34. Laticifse-ous tissue^Is so called, from laiex, tlTO trns- 

 aulritious sap, which it is destined to elaborate and convt y^ It 

 consists of branched anastomosing (ava, to and fro, omftu, % pas- 

 sage) tubes, lying cliiefly in the bark, and the mider fide of 

 leaves. (Fig. 2, c.) 



a. These tubcsvare very iiTeguiar in form, direction, and position:, Tliej 

 expand and contract at intervals, cross and recross the atlicr tissues, and, ii^ccecfe- 

 ing from the inner parts, ramify upon the outer surface, and upon tl 3 hairs, 

 fbnning meshes of inconceivable fineness. Their average diameter •? about 

 nr?n> of an inch. \ They are largest in plants which have a milky latex, nr juices 



35. The EriDERMisf or skin, is a form of cellular tissue exter- 

 nally enveloping the plant. It is found itpon evciy part e: posed 

 to the air, except the stigma of the floAver, and the spot gioles 

 of the roots.; These it does not cover, nor is it found upor those 

 parts which habitually live tmder water. And, where th3 bark 

 of the stem is rugged with seams and furrows, this orgaa is not 

 distinguishable. 



36. It consists of a tissue of flattened cells of various i gures^ 

 filled with air. / Usually there is but one layer of eel; s, but. 

 sometimes there are two or tln-ee, especially in tropical nlants. 

 The Oleander has four. Its office, in the economy of the plant, 

 h, to otieck the evaporation of its moisture. 



a. The delicaie membrane, which may be easily stripped off from the leaf of 

 tlie house-leek or the garden iris, is the epideimis. It is tran.sparent, olorlessy 

 and, under the microscope, reveals its cellular structure. « 



37. The epidermis does not enthely exclude the tissues be- 

 neath it from the external air, but is perforated by ce/tair aper- 

 tures, called STOMATA (mouths), wloich open or close und^r the 

 influence of the hght. (Fig. 2.)") 



38. Stomata are usually of an oval form, bounded by a pais 

 of kidney-shr.ped cells, containing a green matter. Sometimes 

 they are round, and bounded by several cells. Many othes 

 varieties of form have been noticed. 



