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 c 



cumstances, they absorb water from it, and excrete^jwater i 

 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 



