THE STEM 163 



fibres form the part of the rind generally called bast ; it 

 is highly developed in lime trees for instance, and is used 

 for splints, bast-strings, and so on. It is from this bast 

 that all fibres having the structure we have described, 

 wherever they are found, have received the name of 

 bast. The transverse section of an old tree will, there- 

 fore, present the following parts : on the outside the 

 part which we have called the primary cortex ; here, as we 

 shall soon see, a special tissue serving as a protection 

 to the tree is formed at a later period ; then under the 

 primary cortex there is a layer of vascular tissue 

 consisting mostly of bast and sieve-tubes ; we shall 

 call this part the secondary cortex to differentiate it from 

 the primary ; then comes a ring of formative tissue ; 

 nearer still to the centre is the wood, and, lastly, in the 

 very centre the pith. We shall content ourselves with 

 these particulars ; they may have seemed rather tedious, 

 especially when presented in such a necessarily terse 

 form, yet they are indispensable for the understanding 

 of the physiological activity of the stem. Now we 

 enter upon the investigation of the question : what 

 are the paths by which the sap of the plant moves, i.e. 

 by means of which the natural interchange is effected 

 between the substances absorbed by the root and 

 elaborated by the leaves ? 



Let us start with the former, as being the simpler 

 case, that is with the passage of substances from the root 

 towards the aerial parts of the plant, the so-called 

 ascending current. This current furnishes all parts of 

 the plant with the water they require, and in addition 

 with the salts dissolved in it. It is quite easy to dis- 

 cover the path of the moving water, because the want 

 of the necessary quantity of it is generally betrayed 

 by the plant fading. Therefore by making trans- 

 verse incisions at different places on the stem of a living 

 plant, and observing where and when it begins to fade, 



