PHENOMENA DUE TO THE TENSION OF TISSUES. 803 



the pith can also be demonstrated in the case of internodes with solid cylinders of 

 pith which are growing and at the same time increasing rapidly in diameter {e.g. 

 Nicoiiana, Silphium perfoliatum), by dividing a fresh transverse section (laid on 

 glass) through the centre. The two cut surfaces of the pith now become curved 

 outwardly and separate from one another, while the cortical parts of the segment 

 still touch. This is an indication of the outward traction of the pith, and of the 

 tendency of the cortical envelope to dilate peripherally. 



These statements rest however at present on but a small number of observa- 

 tions, and better results may be expected from their repetition. It may nevertheless 

 be assumed that in young internodes, before the fibro-vascular system has begun to 

 become lignified, the pith exerts an outward pressure. This is accompanied later, 

 when the tangential growth of the wood and cortex is more rapid, by an outward 

 traction, which at length becomes so strong as to exceed the tendency of the pith 

 to dilate transversely. The pith is therefore now actually in a state of passive 

 tension transversely (and at the same time compressed longitudinally), until at length 

 the cells in the centre of the pith become detached from one another, and a hollow 

 is formed, if the whole does not lose its sap and become dried up, as for example 

 in the Elder. Kraus observed ^ that the medullary cells of an internode are longer 

 when it is growing than when mature ; but this is to be attributed, in accordance 

 with what has been said, to the loss by the cells of the pith of their power of 

 elongating when isolated. In the internode they are certainly not at first longer, 

 and are afterwards actually shorter ; the difference is only observable on isolation, 

 and indicates that these cells at length lose the property of changing their form when 

 isolated, or in other words become rigid. 



The views here brought forward respecting the tension of the tissues of growing 

 internodes and leaf-stalks are, I think, supported by the fact that the sudden and 

 very considerable lengthening of the pith at the moment of its separation from the 

 surrounding layers of tissue is followed by a slow lengthening which lasts for some 

 days, while, on the contrary, the cortex and epidermis, which are in a state of passive 

 tension, scarcely experience afterwards any perceptible contraction (but, according to 

 Kraus, do not become longer even when placed in water). This subsequent length- 

 ening of the isolated pith takes place with extreme force when it absorbs water, 

 as Kraus has already shown ; but the lengthening also continues in dry air when 

 the pith even loses small quantities of its water, a point which had been previously 

 overlooked. 



The isolated cylinder of pith of a growing internode is very flaccid, flexible, and 

 extensible ; but if placed in water it soon becomes tense, rigid, and elastic, longer 

 and apparently also thicker. The lengthening may amount in a few hours to as 

 much as 40 p. c, or even more. These phenomena are explained if we suppose 

 the contents of the medullary cells to be very strongly endosmotic 2, by which they 

 become in a high degree turgid, and thus not only increase considerably in size. 



* Bot. Zeitg. 1867, p. 112. 



"^ Notwithstanding this powerful endosmose, the amount of solid substance dissolved in the 

 cell-sap of the parenchyma is very small, as is shown by the fact that in cylinders of pith of this 

 kind I found the dry weight only from 2 to 5 p. c, a considerable portion of which belonged to the 

 cell-walls and protoplasm. 



3 F 2 



