548 



mechanical actions. In these cases, as in the others, 

 that ordinarily become dense, deviate from this typical character 

 when they are not exposed to those forces which produce dense 

 tissue by increasing the extravasation of sap. 



To complete the proof that such a relation exists, let me add the 

 results of some experiments on equal and similarly-developed parts, 

 kept respectively at rest and in motion. I ha,ve tested the effects on 

 large petioles, on herbaceous shoots, and on woody shoots. If two 

 6uch petioles as those of Rhubarb, with their leaves attached, have 

 their cut ends inserted in bottles of dye, and the one be bent back- 

 wards and forwards while the other remains motionless, there arises, 

 after the lapse of an hour, scarcely any difference in the states of 

 then* vessels : a certain proportion of these are in both cases charged 

 with the dye, and little exudation has been produced by the motion. 

 Here, however, it is to be observed that the causes of exudation are 

 scarcely operative ; the vascular bundles are distributed all through 

 the mass of the petiole, which is formed of soft watery tissue ; and 

 they are, therefore, not so circumstanced as to be effectually com- 

 pressed by the bends. In herbaceous stems, such as those of the 

 Jerusalem Artichoke and of the Foxglove, an effect scarcely more 

 decided is produced ; and here, too, when we seek a reason, we find 

 it in the non-fulfilment of the mechanical conditions ; for the vascular 

 bundles are not so seated between a tough layer of bark and a solid 

 core as to be compressed at each bend. When, however, we come 

 to experiment upon woody shoots, we meet with conspicuous effects, 

 though by no means uniformly. In some cases oscillations produce 

 immense amounts of exudation parallel transverse sections of the 

 compared shoots showing that where, in the one that has been at 

 rest, there are spots of colour round but a few pitted ducts, in the 

 one that has been kept in motion the substance of the wood is soaked 

 almost uniformly through with dye. In other cases, especially where 

 there is much undifferentiated tissue remaining, the exudation is not 

 very marked. The difference appears to depend on the quantity of 

 liquid contained in the shoot. If its substance is relatively dry, the 

 exudation is great ; but it is comparatively small if all the tissues are 

 fully charged with sap. This contrast of results is one which con 

 templation of the mechanical actions will lead us to expect. 



And now, with these facts to aid our interpretation, let us return 

 to ordinary stems. If the upper end of a growing shoot, the prosen- 

 chyma of which is but little thickened, be allowed to imbibe the dye, 

 the vessels of its medullary sheath alone become charged ; and from 

 them there takes place but a slow oozing. If a like experiment be 

 tried with a lower part of the shoot, where the wood in course of 

 formation has its inner boundary marked but not its outer boundary, 

 we find that the pitted ducts, and more especially the inner ones, 

 come into play. And then lower still, where the wood has its peri- 



