TISSUE-TENSIONS. 



569 



of the passive extension of the cell-wall for its proper growth — as I had previously 

 concluded on other grounds, was demonstrated by De Vries particularly by the estab- 

 lishment of the proposition, that ' the rapidity of the growth in length in the partial 

 zones of growing organs rises and falls with the magnitude of the extension caused 

 by turgescence.' That is to say, the extension due to turgescence first increases in 

 passing backwards from the apex of a shoot or of a root, attains its maximum in the 

 region of most active growth, and diminishes thence towards the base, just as does the 

 partial growth ; at the boundary between the fully-grown and the growing region lies 

 also the limit of extension due to turgescence. In order to comprehend this matter 

 quite clearly, it is necessary to refer to what I said in the preceding lecture concerning 

 the distribution of growth — the partial growths. Finally it need only be mentioned that 

 the contraction of a growing organ in the salt-solution, and its subsequent re-extension 

 in pure water, is to be regarded in the first place only as the measure of the existing 

 extension of the growing cell-walls, but not as a measure of the magnitude of the 

 force of turgescence. This is at once clear on again referring to a non-cellular 

 plant — e. g. the Pliycojnyces already mentioned so often ; evidently the turgescence 

 in the much branched utricle of which this plant consists is everywhere equal in 

 magnitude, though extension of the membrane is only actually taking place at the 

 growing ends of the utricle. 



So much at any rate follows definitely from the considerations and facts so far, 

 that vegetable cells only grow when their cellulose walls are passively extended by the 

 pressure of the cell-sap, and therefore that this extension is an essential mechanical 

 cause of the growth itself. 



It appears, however, that in certain layers of tissue in the more highly organised 

 plants, the passive extension which is necessary for growth may also be produced in 

 another way, as we might conclude from the fact of the tension of the tissues, which 

 has also already been described. From the phenomena which result on separating 

 the various layers of tissue of a growing shoot-axis, it follows that the epidermis, as 

 well as the vascular bundles and the as yet non-lignified strands of sclerenchyma, 

 are subjected to continual passive extension, due to the much more rapid growth 

 of the soft parenchyma, especially the pith. Although some points still remain 

 doubtful, we may nevertheless ^.ssume that this passive extension of the epidermis, 

 vascular bundles, and strands of young sclerenchyma by the growing parenchyma, 

 is to be looked upon as an important factor in the growth of these passively 

 extended portions of tissue. 



We previously noticed the tensions of tissues only in so far as they affect 

 the rigidity and elasticity of the young growing shoot-axes and leaves, and it 

 has only to be added to this that the rigidity even of many fully-grown parts, par- 

 ticularly of petioles and the ribs of leaves of the Dicotyledons, depends even in the 

 fully-grown condition on these tissue-tensions. 



Now, however, we have to regard the tissue-tensions as a mechanical 

 property of the growing organs of multicellular plants, and to see in what relation 

 they stand to growth itself. In the first place, so much is certain, that the parts 

 of plants concerned only grow at all so long as tissue-tensions exist in them : 

 if these are destroyed by doing away with the turgescence in the parenchymatous 

 tissue, the growth ceases also, and the more pronounced the turgescence of that 



