344 



LECTURE XV. 



In other cases the various tissues were separated from each other and 

 the variation of their length determined, as in the following instance. 

 The length of the internode is taken as 100, and the variations in length 

 are estimated as percentages, shortening being indicated by a minus 

 sign, and lengthening by a plus sign. The plant in the case given was 

 Nicotiana Tabacum. 



The following general expression for the relative lengths 

 of the tissues after isolation has been given by Sachs, where 

 E, C, V, P, stand respectively for epidermis, cortex, vascular 

 tissue, and pith : 



E<C<V<P>V>C>E. 



This expression also states the relative active tension 

 (compression) of the layers, for the greater the compression 

 the greater will be the elongation of a tissue on isolation. The 

 expression for the passive tension (stretching) will be 



E>C>V>P<V<C<E. 



The most general statement of the distribution of the longi- 

 tudinal tension in stem-structures is that, passing from the 

 centre to the surface, each of the layers of tissue is stretched 

 by the layers internal to it, and is compressed by the layers 

 external to it. 



In the case of roots the longitudinal tension is much less 

 considerable. If the growing apex of a root be slit vertically, 

 the two halves remain at first in contact : it is only after lying 

 for some time in water, and in consequence of subsequent 

 growth, that any curvature becomes perceptible. It is then 

 of this kind, that the two halves remain in contact at the 

 apex, but bend away from each other, so that their external 



