724 MECHANICAL LAWS OF GROWTH. 



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

 traction, which at length becomes so strong as to overcome 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 very 

 slightly longer when it is growing than when mature ; but this may be explained, in 

 accordance with what has been said, by the cells of the pith finally losing their power 

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

 and are afterwards shorter ; but 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 have 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 medullary 

 cells to be very strongly endowed with endosmosc^, by which they become in a high 

 degree turgid, and thus not only increase considerably in size, but also become more 

 rigid. The considerable increase in size presupposes, however, from the rapidity 

 with which it takes place, great extensibility in the cell- walls. Isolated prisms of 

 pith exposed to the air become shorter even than the length they possessed in the 

 internode^; the cell-walls which were previously in a state of tension evidently 

 contract elastically, as the turgidity diminishes from loss of water. 



But if care is taken that isolated cylinders of pith do not absorb any water, 

 while at the same time they can only lose a very small quantity of it, by enclosing 

 them in a glass tube containing about i litre of dry air, they nevertheless continue to 

 lengthen perceptibly for some days, although not so considerably as when they 

 absorb water; and this lengthening affects chiefly the older parts, while the 



' Bot. Zeitg. 1867, p. 112. 



2 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. 



" Kraus /. c, Tables, p. 29. 



