THE FORMATION OF WOOD IN PLANTS. 415 
To eomplete the proof that such a relation exists, let me add {he results of some 
experiments on equal and similarly-developed parts, kept respectively at rest and in 
motion. I have tested the effects on large petioles, on herbaceous shoots, and on woody 
shoots. If two such petioles as those of Rhubarb, with their leaves attached, have their 
cut ends inserted in bottles of dye, and the one be bent backwards and forwards while 
the other remains motionless, there arises, after the lapse of a hour, scarcely any differ- 
ence in the states of their 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 eonditions; 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 compara- 
tively small if all the tissues are fully charged with sap. This contrast of results is one 
Which contemplation 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 prosenchyma 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. Ifa 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 periphery 
defined and its histological characters decided, the appearances show that the tissue 
forming its outer surface begins to take a leading part in the transmission of liquid. 
What now is the explanation of these changes, mechanically considered? In the young 
Soft part of the shoot, as in all normal and abnormal growths that have not formed 
m the channels for the passage of sap are the — Media: oe; 
: Vessels. These vessels, here included in the bundles o 
‘n common with the tissues around them, subject, by the bendings of the shoot, to slight 
intermittent compressions, and, especially the outermost of them, are € to 
give the prosenchyma an extra supply of nutritive liquid. The thickening of the pros- 
Mehyma, spreading laterally as well as outwards from each bundle of the medullary 
M 
