BOTANY 



as the water-courses in plants are all completely shut off from the outer atmo- 

 sphere, the external atmospheric pressure could have no effect. The rarefied air 

 within the plants, moreover, shows no such regularity in its distribution that.it 

 could possibly give rise to so continuous a flow of water. Further, as the 

 atmospheric pressure can only sustain the weight of a column of water 10 m. high, 

 while the sap of a Begonia ascends 60-100 m., the inadequacy of the atmospheric 

 pressure to give rise to such a movement must be admitted. 



The supposition that the water ascends in the form of vapour through the 

 cavities of the wood, and is afterwards condensed in the leaves, is untenable, as is 

 at once obvious from a consideration of the anatomical structure of the wood, the 

 interruption of its cavities by short columns of water, and the temperature 

 of the plants themselves. And, moreover, the special task of the transpiration 

 current, to transfer the nutrient salts, could not be accomplished if such a supposi- 

 tion were true. 



It has also been suggested that all of these processes might be aided by THE 

 oo-opekation or the livihg cells which are so abundant throughout the wood, 

 and which have command of active osmotic forces, to the service of which they 

 could unite a regulative irritability. Later investigations, however, have shown 

 that poisonous solutions, which would at once kill all living protoplasm, are 

 regularly transported, in great quantities, to the summits of the loftiest Oaks 

 and Firs. Thus the supposition that the living elements in any way co-operate 

 in the ascent of the transpiration current is absolutely precluded. 



The view most generally accepted at the present time, that THE 



TRANSPIRATION CURRENT ASCENDS IN THE CAVITIES OF THE WOOD 



through the vessels and tracheids, seems to be supported by- 

 observation as well as by the structural features of the wood, but 

 leaves the question as to the cause of the movement still unanswered. 



Sachs, in his THEORY OF IMBIBITION, sought to solve the problem by 

 supposing that the water ascended in the substance of the lionifibd 

 walls, and that the upward movement was due to the force of molecular attrac- 

 tion, and to the disturbance of the equilibrium existing between the water and 

 the substance of the cell walls. 



In more recent attempts to account for the ascent of the sap, the direct transfer 

 to the root cells of the force of suction arising from the transpiring green leaves, 

 has been regarded as resulting from the internal cohesion of the water itself. On 

 such a supposition, however, no evidence is furnished that the suction would, in 

 itself, be sufficient to induce a movement like that of the transpiration current. ' 



The Giving-off of Water. — The requisite amount and essential 

 concentration of the nutrient water supplied by the transpiration 

 current are maintained only by the constant discharge of the accumu- 

 lating water. This may occur in two ways, either more profusely by 

 the evaporation of the water through the cell walls in the form of 

 vapour — that is, by transpiration — or less copiously and also less fre- 

 quently by the actual exudation of drops of water. 



I. Transpiration. — In their outer covering of cork, cuticle, and 

 wax, plants possess a protection from a too rapid loss of water. A 

 Pumpkin, with its thick cuticle and outer coating of wax, even after it 



