2l6 LECTURE XIII. 



If a large leaf-stalk from a Rhubarb plant, for instance, or a Hcracleum, &c., 

 or even a portion of the growing flowering stem of these plants, is cut off, an 

 excellent object is to hand for making clear the question here brought forward. Let 

 us suppose the object cut off square above and below, and its length to be about 

 50 cm. If now a strip of the epidermal tissue together with the collenchyma layers 

 which strengthen it are removed completely, and the attempt is then carefully made 

 to lay this collection of tissues again in its place, it is observed that the epidermal 

 strip is too short : it has become elastically contracted during the separation, and 

 thus, in the natural condition of the object, had been passively extended. If the whole 

 epidermis is now removed all round, and the length of the very succulent cylinder 

 (which consists chiefly of parenchyma and very extensible vascular bundles which 

 scarcely come into consideration here) is measured, it is found to have increased 

 very considerably in length during the manipulation \ Not rarely, such a cylinder 

 becomes extended from 50 cm. to 53 or 55 cm., or even more. In the natural 

 condition, where the epidermis enveloped the succulent cylinder of tissue, the latter 

 was thus passively contracted, and had a tendency to become extended ; it was 

 prevented from so doing, however, by the elasticity of the epidermis and collen- 

 chyma. There existed, therefore, in the natural condition of the whole, a mutual 

 tension between the epidermal tissue and the succulent fundamental tissue; the 

 latter behaved to a certain extent like the contents of a turgescent cell, which 

 distend the membrane. But of course it is not to be supposed that in the passive 

 compression of the tissue, it depended upon the compression of the water con- 

 tained in it, since this is, as regards forces coming into consideration here, simply to 

 be regarded as not compressible. The lengthening of the peeled cylinder of tissue 

 depends rather, as we shall see later, on a sudden alteration in the form of its cells — 

 they become longer and narrower. Nevertheless, the comparison is apt in other 

 respects, since it can be shown that in the natural objects also a transverse tension 

 exists, of such a kind that the inner tissue exerts a pressure on the surrounding 

 epidermal tissue in the transverse direction also. INIoreover, this condition of 

 so-called tissue-tensions is only found when the objects named are abundantly 

 supplied with water : if they had previously been allowed to droop through loss of 

 water, the separation of the masses of tissue would only give inconsiderable dif- 

 ferences in length between the epidermis and the internal body of tissue, or even 

 none at all. 



We must now, however, regard yet another point in our simple experiment. 

 The leaf-stalk or portion of stem in the fresh state, or at any rate after having 

 been previously submerged for some hours in water, was tense and stiff: it 

 possessed considerable elasticity and rigidity. The removed strips of epidermis, 



' The alterations in dimension taking place on the separation of tissues from one another were 

 first scientifically established by Brücke in 1848 in his ' Untersuchtmg über die Bewegung der 

 Mimosen ' on the motile organs of the latter. In my investigation, ' Über das Bewegungsorgan und 

 die periodischen Bnvegungen der Blätter von Phaseolus und Oxa/is' (Bot. Zeitung, 1S57), I treated 

 of the same phenomenon. The views on the tensions of tissues were lat^r, however, led in a wrong 

 direction by various publications of Hofmeister, since he chiefly observed the cell-walls only, and 

 did not pay sufficient attention to the pressure between the cell-sap and the wall. I then (in my 

 'Lehrbuch der Botanik,' especially in the III and IV editions) again brought the theory of the 

 tensions of tissues, as a consequence of the turgescence of cells, into the direction previously pursued. 



