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. 
Nicotiana^ Silphium perfoliatuni)^ 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 ^, by which they 
become in a high degree turgid, and thus not only increase considerably in size. 
1 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 
