CAUSES OF THE MOTIOlir OP 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 Tcnsioilt — 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-multiplioation — is 

 taking place. In the simple cell similar tension may exist, 

 caused by the tendency of the fonnative layer to expand 

 beyond the limits of the cell-wall. Another cause of 

 tension is the different imljibing and osmotic power of the 

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