PHENOMENA DUE TO THE TENSION OF TISSUES. 723 



become wider in diameter, while those of the pith which lengthen must on the con- 

 trary become narrower. It is impossible however to measure directly these changes 

 of diameter, which are so small that ordinary methods are inapplicable. 



It is, however, a necessary consequence of what has been said that the passive 

 lengthening of the epidermal cells, &c., in a growing internode makes them narrower ; 

 the young epidermis must therefore be too narrow, besides being too short for the 

 inner masses of tissue. Similarly the pith, being prevented from elongating in the 

 growing internode by the surrounding layers, must in consequence have a tendency 

 to enlarge transversely; besides being too long for the elongated tissues, it will 

 also be too thick for them, and must have a tendency to force them apart. It 

 follows therefore from the longitudinal tension which has been observed in the 

 layers of tissue of a growing organ, that a transverse tension must also exist in it 

 of such a nature that the outer layers are in a state of passive tension, w^hile the 

 medullary cells which are prevented from lengthening have a tendency to dilate 

 transversely. 



If thin transverse segments^ are cut radially from somewhat older growing stems, 

 they gape open, evidently because the epidermis contracts in the peripheral direction, 

 having been previously of too small circumference for the inner tissue, in other words, 

 in a state of passive tension. The tendency of the medullary cells which are pre- 

 vented from lengthening to become broader transversely, does not appear, on the 

 other hand, to be always hindered by the surrounding wood and cortical tissue, but 

 often to be even promoted by them ; so that these layers of tissue which surround 

 the pith grow more rapidly in the peripheral direction than does the pith itself, and 

 therefore exercise a radial traction upon it. A striking proof of this phenomenon is 

 afforded by the frequent formation of cavities in stems and leaf-stalks at the time 

 and place where the growth in length is most rapid. The increase in thickness of 

 the pith is not sufficient to fill up the space which is enclosed by the surrounding 

 tissues, and which increases in size ; its cells separate in the longitudinal direction, 

 and the woody cylinder remains clothed on the inside by a layer of pith, the longi- 

 tudinal tension of which still continues. The existence of an outward traction upon 

 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 per/oliaiu77i), by dividing a fresh transverse segment (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 observ- 

 ations, 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, 



permeated with water alter their volume from the forces exerted in this case. An alteration in the 

 volume of the entire pith could at most arise from a change in the size of the intercellular spaces 

 in consequence of the change in form of the cells. 

 ^ Sachs, Experimental-Physiologic, p. 471. 



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