CAUSES OF THE MOTION OF JUICES. 373 
rhubarb (pie-plant,) we shall notice that it curves to a coil 
or spiral. This skin consists of the true cuticle with a 
coating of cell-tissue adhering. The tension of the latter 
and the passivity of the former occasion the curvature. 
Further dissection demonstrates that in general the cuti- 
cle, the wood-cells, and the vascular bundles, are passive, 
while the cell-tissues of ‘the rind and pith, and the corre- 
sponding cell-tissues of the leaves, are tense. 
It follows from these considerations that the length of a 
fresh growing stem must be different from the length of 
its parts when separate from each other. If we divide a 
succulent stem lengthwise, into the pith, the wood and 
the rind or the corresponding parts, and accurately . 
measure them, we shall find in fact that they differ as to 
length from each other and from the stem as a whole, 
The pith, when the wood is cut away, elongates, the wood 
shortens, the rind shortens still more. In the original 
stem the cell-tissue being united to the vascular, stretches 
the latter and is at the same time restrained by it. On 
their being cut apart, the one is free to extend and the 
other to shorten. Sachs gives the following comparative 
measurements of the stem of a tobacco plant, and of its 
parts after separation—the length of the stem being as- 
sumed as 100: 
‘Entire stem - -  - 100 
Rind - - - - 94,1 
Wood - - 98.5 
Pith » = 102.9 
Causes of Tension.—This tense condition of the con- 
siderably developed stem depends partly upon the unequal 
nutrition of the different tissues. Those parts, in fact, ex- 
ert tension in which rapid growth—cell-multiplication—is 
taking place. In the simple cell similar tension may exist, 
caused by the tendency of the formative layer to expand 
beyond the limits of the cell-wall. Another cause of 
tension is the different imbibing and osmotic power of the 
