194 BOTANY faut i 



cold window pane. Transpiration can be very strikingly demonstrated by the 

 change in colour of cobalt paper ; filter-paper soaked in a solution of cobalt-chloride 

 has when completely dried a blue colour which changes to red on the presence of 

 water. If a small piece of this cobalt paper is laid on a leaf and protected from 

 the dampness of the atmosphere by a slip of glass, the cliange in colour to red that 

 commences at once indicates the transpiration ; conclusions as to the quantity of 

 water given off may be drawn from tlie greater or less rapidity of the commencement 

 and progress of the change in colour. It is true that exact information on this 

 point can only be obtained by weigliing experiments. These show that the loss of 

 water vapour by a plant is usually so great as to be recorded as a common balance 

 without great difficulty in the course of a quarter of an hour. No general statement 

 can be made as to the amount of transpiration from a unit area of transpiring 

 surface, for this depends on many external factors, e.(j. temperature, light, supply of 

 water, etc., as well as on the structure of the plant. 



The process of transpiration takes place in this way : An 

 epidermal cell exposed to the air will lose some of the imbibition 

 water of its cell-wall by evaporation ; this would go on until the 

 cell-wall was dried by the air if a reserve of water were not obtainable 

 from within the cell. This is in fact obtained from the protoplasm, 

 from which the cell-wall no longer fully saturated, withdraws imbibi- 

 tion water, and the protoplasm in turn makes good its loss from the 

 vacuole. Thus the movement of the water affects the interior of 

 the cell, and brings about a concentration of the cell-sap. Thus the 

 conditions are established for the cell to absorb water from an adjoin- 

 ing cell which is not itself transpiring, and the loss of water is thus 

 conducted from the superficial cells where evaporation is taking place 

 into the depths of the tissue. The amount of transpiration primarily 

 depends on the permeability to water of the cell-wall ; this is closely 

 connected with the power of the wall to take up water, or the amount 

 of imbibition. If the cell-wall is an ordinary cellulose membrane the 

 amount of transpiration will be large ; when the wall is covered with 

 wax or cuticle, or impregnated with cuticular substance, it both absorbs 

 and gives off little, water. Comparative investigations on suitable 

 objects, by means of cobalt paper, show how the transpiration diminishes 

 with the increase in thickness of the cuticular layers until it ultimately 

 becomes practically non-existent. Corky walls behave in the same 

 way as cuticularised layers. 



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 has been separated 

 from its parent plant for months, suffers no great loss of water. A 

 potato or an apple is similarly protected by a thin layer of cork from 

 loss of water by evaporation. The green organs of plants, on the 

 other hand, which must be able to get rid of the surplus water in 

 order to secure the concentration of the nutrient salts and to reduce 

 their temperature, make little use of such protective coverings. On 

 the contrary, they are provided, besides the adaptations to regulate the 



