ABSORPTION OF FOOD BY PLANT 359 



better channel available, which will naturally come into 

 requisition where quick transit is required. 



In a woody trunk, then, we have (i) the outer cortical 

 cylinder of water-conducting tissues, by which the ascent of 

 sap takes place slowly. We have (2) the highly-conducting 

 central woody tissue, which not only allows of water ascend- 

 ing rapidly through it, but is also (3) in lateral communi- 

 cation with the outer cylinder. The hydraulic system thus 

 consists of a large central canal, as it were, connected with 

 innumerable lateral reservoirs, which are the cells of the 

 cortex. When a demand arises for rapidity of water-supply 

 on account of transpiration, we can now see that no less than 

 three different factors are brought into requisition. First 

 there is the rapid upward transit through the wood ; 

 secondly, the slow ascent through the cortex ; and thirdly, 

 the lateral supply from the cortex by way of the nearest 

 wood. 



As regards the last of these, the cortical tissues in contact 

 with the wood act in a manner not very unlike that of the 

 roots towards the soil. That is to say, under different cir- 

 cumstances, they absorb water from it, and excrete water 

 into it, these alternating processes being by no means 

 accidental, but guided by appropriate excitatory reactions. 



Turning our attention for a moment to the movements 

 of Mimosa leaf, we find that on excitation the expelled water 

 makes its way to the fibre-vascular tissue. There is here, in 

 the excitable tissue, unlike the case of secretory organs, no 

 external vent, and we see the necessity of a central reservoir 

 to which water excitatorily expelled may find access. On 

 the subsidence of excitation, the water is re-absorbed by the 

 organ, causing expansion and re-erection of the leaf. Such 

 movements of inflow and outflow evidently take place in the 

 trunk of the tree itself. Under the stimulus of sunlight, 

 the excited cortical tissue will squeeze water inwards into the 

 central reservoir. If this takes place, the effect will be seen 

 in a diametric contraction. At the time when transpiration 

 is most rapid, under the action of sunlight, there is thus 



