358 COMPARATIVE ELECTRO-PHYSIOLOGY 



only one or two days old, again, water ascends, and is ex- 

 creted at the tip of the yet unopened leaf. Transpiration 

 is here at its minimum, and the fibre-vascular elements at 



.this early stage cannot be regarded as ' dead wood.' Finally, 

 in herbaceous plants, where woody elements are insignificant, 

 the ascent of sap is seen to take place. 



Assuming, for the sake of argument, that in the case of 

 the tree, the mass of wood in the interior were dead, it might 

 still conceivably be of use in the irrigating system as a 

 central reservoir. This would certainly be advantageous to 

 rapidity of transit. At the lower end of the tree, the wood 

 abuts upon the delicate parenchymatous tissues of the root, 

 and at the upper upon those of the leaves. According to 

 the physiological theory, then, it might be supposed that it 

 was by the multiple activity of the cells of the root that 

 water was pumped into the wood ; and that at the other end 

 the central reservoir was able to furnish a supply to make 

 up for the constant loss by transpiration. Laterally also, in 

 the stem itself the cortical tissues could draw upon this 

 central supply. Under such an arrangement no part of the 

 plant could be very far away from the reservoir. 



As a matter of fact, this sketch corresponds roughly to 

 the working of the tree as an hydraulic machine. The 

 system is, however, somewhat more complex than has been 

 indicated. Besides the central, we have also to remember 

 the presence of lateral reservoirs, in the parenchymatous 

 tissues of the cortex. But tlie transport of water through 

 these is not, of course, so rapid as through the central, more 

 specifically conducting, system. In the case of herbaceous 

 plants, where the quantity of wood is insignificant, we may 

 regard the central channels as abolished. Here we have 

 soft cortical tissues extending continuously from root to 

 leaves through the stem, and it is obviously through these 

 that the ascent of water takes place. In woody trees, then, 

 there is no reason to suppose that the cortical tissues could 

 not play a similar part in the conveyance of water. The 

 difference is, that in this case there is also an added and 



