DIV. ii PHYSIOLOGY 229 



1. If a transpiring part of a plant is covered with a bell-jar that has been pre- 

 viously cooled, the water vapour given off from the plant will be deposited in the 

 liquid form on the inside of the bell-jar, just as the aqueous vapour in our breath 

 condenses on a cold window pane. 2. 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 

 change in colour to red, that commences at once, indicates the transpiration ; 

 conclusions as to the quantity of water given off may be drawn from the greater 

 or less rapidity of the commencement and progress of the change in colour. 

 3. Exact information on this point can only be obtained by weighing 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.g. 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. 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. 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 gives off little water. Com- 

 parative 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 



