LECTURES ON BOTANY. 



119 



This last fonii of the vegetable elementary 

 organs consists of one or more filaments coiled 

 spirally within a very delicate membraneous 

 tube, similarly attenuated toward either end 

 as are those of the wood and the duct, but, 

 unlike the spirals apparent m some of the 

 latter, capable of being drawai out like a 

 spring, when the part containing them is 

 broken. They are very advantageously seen 

 by breaking carefully across the leaf of a 

 rose or sti-awberry, or the young brittle shoot 

 of the rose, and drawing the parts slowly 

 asunder, as exhibited in tig. 3. The spiral 

 coils are in most instances so close together 

 as to appear to form the tube, and the mem- 

 brane which invests them is so exceedingly 

 fragile as to break })etween each coil of 

 the spire when it is drawn out, so as not 

 to be readily discernible unless occasional- 

 ly when the extremity of the filament is 

 relaxed, as represented in one of our fig- 

 ures. Each spiral is generally composed of 

 a single filament or tlu'ead, but ui some 

 plants the number of parallel fibres twined 

 in the same direction is considerable; one 

 of the figures exhibits a magnified view of 

 a portion of such a vessel from the stem of 

 a banana, consisting of three threads or fibres ; 

 but in this, and many other plants of the same 

 and allied orders furnished wth compound i 

 vessels, there does not appear to be any reg- 1 

 ularity in their production. De Candolle re- | 

 marks that the niunber ot threads composing 

 each spiral of the plant just i-eferred to va- 

 ries from seven to twenty-two ; but they are 

 sometimes formed of a single thread, and I 

 have found in the saine portion of the stem 

 no fewer than eleven different modifications 

 of these curious vessels, varying in the num- 

 ber of their threads from two to twenty-nine, 

 the largest number which has, to the best of 

 my knowledge, been met with in any ulant. 

 The size or diameter of the spiral tube Is ex- 

 ceedingly variable, the largest being about 

 the three-hundredth or four-hundredth of an 

 inch, while jn some cases they are not above 

 the two-thousandth or three-thousandth. They 

 are variously distributed, but chiefly in the 

 young stems or shoots, in which they occupy 

 that cylinder of woody substance that inmie- 

 diately sui-rounds the pith, called the medul- 

 lary sheath, and in the stalks and veins of the 

 leaves and other organs which are modifica- 

 tions of them and originate from the sheath in 



■ question. They are of very rare occurrence 

 in the root, and still more so in the bark and 

 the true wood, or that which in after periods 

 of growth forms around the first year's layer. 

 Where the tissue of the stem is not stratified, 

 and the i)ilh or cellular substance does not 

 form a separate cylinder or colunni in its cen- 

 ter, as in the liliaceous orders, palms, and 

 others belonging to the same great natural 



•class, the spiral vessels accompany the l)un- 



■ dles of woody fibre and ducts that lie dis- 

 persed througli its mass, and are present oiten 

 in such abundance as to constitute the most 

 remarkable feature in their internal structure; ; 



from the stems and bases of the large leaves 

 of the banana and plaiiitain they may be 

 drawn out by the handtull, and in the West 

 Indies are sometimes collecled iu this way 

 for tinder. 



The use of the spiral vessels in the vegetable 

 economy is, like that of their other minute or- 

 gans, rather to be surmised than demonstrated. 

 A very general opmion, from tlie earhest peri- 

 od of their discovery, regarded them as organs 

 of respiration ; hence their denominations of 

 "trachea" aud " trachenchyma," alluding to 

 their sujiposed correspondence in function 

 with the trachea (or windpipe) and air-tubes 

 in the lungs of an animal ; to those of insects 

 : their structure presents a remarkable parallel. 

 I They are generally filled only with air ; aud 

 although instances may occur in which they 

 are " gorged with fluid," such instances are 

 I rare, and probably either accidental, or arising 

 I from one of those morbid changes to which 

 the minuter portions of the vegetable fab- 

 ric are unquestionably liable, although their 

 I causes are hidden in consequence of our 

 imperfect knowledge and slender means of 

 j inquiry. Tlie opinion, however, founded only 

 I upon analogies that may be rather fancied 

 than real, seems of late years to have been 

 j gi-adually losing ground in the estimation of 

 j some of the best physiologists, although they 

 have been hitherto incapable of substituting a 

 more i)lausible hypothesis ; and the principal 

 argument that can be adduced against that pre- 

 viously entertained, consists in the fact that 

 the spiral vessels of plants are not in immedi- 

 ate connection with the surface pores or sto- 

 mata, nor even with the air-chambers with 

 which the latter communicate, and hence dif- 

 fer i'rom those accompanying the breathuig 

 apparatus of an insect. When, however, we 

 reflect upon the penneability of the vegetable 

 membrane to a comparatively gross fluid like 

 tlie sap, the passage of one so thin and infi- 

 nitely more diffusible as air can scai'cely be 

 denied, even though it had to penetrate to 

 a much greater depth than to the ulterior of 

 a leaf, an organ the stmcture of which is so 

 strictly accordant with its supposed and al- 

 most experimentally-proved function as the 

 vegetable lung. 



Under whatever conventional name the 

 elementary organs are known, however gi-eat 

 the apparent difierence of their structure, and 

 diversihed their functions or ofiices, the deep 

 aud unwearied researches of modern naturalists 

 aided by the increased jiowers of ol>servation 

 placed at their command by the imj)rovement 

 of the microscope, have led to the conclusion 

 that they are all only so many modifications 

 of the cell, while the wall of the cell itself 

 is probably merely consolidated mucus, as- 

 suming the form of membrane or fibre, ac- 

 cording to laws of which we are at present 

 ignorant. That changes take place of one 

 kind of tissue into anotlier, at difierent stages 

 of the growth of the plant is unquestionable, 

 as well as that such clianges are correspond- 

 ent witli alteration of function ; but it is no less 



