but by the bounding walls of those cavities. The 

 action of wind swaying the leaves causes an in- 

 creased amount of transpiration to take place, and 

 in this way affects the upward current. 



Thus far we have dealt with the course and movement of the 

 ascending current of water through the plant, maintained, as 

 we have have seen, in obedience to the pull given by trans- 

 piration, the fluid being supplied by the absorptive power of 

 the parenchyma cells of the root. 



Now we have to speak shortly of what was formerly known 

 as the " descending current of the sap" or " the elaborated sap." 

 It was believed, and is still believed by many, that the ascending 

 current of water, after being deprived in the leaves of a large 

 portion of its water by evaporation and transpiration, descends 

 again, taking a definite course after having received a new 

 element into its composition, which substance is of the highest 

 importance in the growth of the plant— viz., Carbon. This 

 Carbon is obtained from the small quantity of the carbon 

 dioxide gas which is present in the air, and which is taken up 

 principally, as appears from the recent experiments of Boussin- 

 gault, of which mention has been already made, through the 

 cuticularised cell-walls of the epidermal cells covering the upper 

 surface of the leaf, and not by means of the intercellular passa- 

 ges of the lower surface, and the stomata which communicate 

 with them, as was formerly believed to be the case. 



The green colouring matter, chlorophyll, so almost univer- 

 sally distributed in the vegetable kingdom, is present in the cells 

 of the leaf parenchyma ; and under the presence of it, of an 

 adequate supply of air containing the carbon dioxide, of bright 

 sunlight, and of certain appropriate salts, more particularly salts 

 of potassium, the plant is able to separate the carbon from the 

 carbon dioxide, and to combine it with oxygen and hydrogen in 

 the proportions in which the latter occur in water, and thus 

 form the carbo-hydrate well-known as starch. The formation 

 of the starch takes place only in connection with the living 

 protoplasm of the cell, and the part of it specially set apart 



