208 BOTANY PAKT i 



give off two million litres of water in four months, and an acre of 

 hops three to four millions. For a Birch tree with about 200,000 

 leaves and standing perfectly free, VON HOHNEL estimated that 500 

 litres of water would be lost by evaporation on a hot dry day ; on an 

 average the amount would be 60-70 litres. A hectare of Beech wood 

 gives off on the average about 30,000 litres daily ( 29 ). 



It has been calculated that during the period of vegetation the 



Beech requires 75 litres and the Pine only 7 litres for every 100 



grammes of leaf substance. According to DIETRICH, for every gramme 



of dry, solid matter produced, on the average, 250-400 grammes of 



" water are evaporated. 



EXPERIMENTAL DEMONSTRATION OF TRANSPIRATION. The evaporation from 

 plants, although imperceptible to direct observation, may be easily demonstrated, 

 and its amount determined by the help of a few simple appliances. One method 

 r of doing this, is to weigh a plant before and after a period of vigorous evaporation, 

 and thus determine the amount of water actually lost. Or, if the water evaporated 

 by a plant placed under an air-tight bell-jar be absorbed by calcium chloride or 

 concentrated sulphuric acid, it will only be necessary to determine the increase in 

 weight of the absorbing substance to estimate the amount of water given otf by 

 evaporation. The amount of water taken up by a plant may also be shown by so 

 arranging the experiment that the water passes in through a narrow tube, as then 

 even a small consumption of water will be quickly indicated by the rapid change 

 of the water-level, which will be the more rapid the smaller the bore of the tube. 

 An apparatus of this kind is called a potetometer, and by its means the amount 

 of water taken up by a transpiring branch may be determined. 



The important part taken by the stomata in the process of transpiration may be 

 easily shown, according to STAHL, by means of the cobalt reaction, or the change 

 in colour of dark-blue dry cobalt chloride to light rose upon absorption of water. 

 In making this experiment a leaf placed between strips of paper which have been 

 previously saturated with this cobalt salt and then thoroughly dried, is laid between 

 glass plates. The paper on the side of the leaf most abundantly supplied with 

 stomata will then first change its colour, and that too the more rapidly the more 

 widely open are the stomata. The cobalt reaction, as also the iron and palladium 

 chloride reaction used by MEUGET, may be used to determine variations in the 

 width of the stomatal openings. FR. DAUWIN used a delicate hygrometer for this 

 purpose in order to follow continuously the variations in width of the opening. 



By the dimming of a film of collodion through which all the microscopic detail 

 of the leaf surface could be seen, BUSCALIONI and POLLACCI were able to prove that a 

 slight general transpiration in addition to that through the stomata takes place ( 30 ). 



It is evident from these and similar experiments that more water is evaporated 

 in a given time from some plants than from others. These variations are due to 

 differences in the area of the evaporating surfaces and to structural peculiarities 

 (the number and size of the stomata, presence of a cuticle, cork, or hairy covering, 

 etc.). But even in the same shoot transpiration is not always uniform. This is 

 attributable to the fact that, both from internal and external causes, not onlv tliu 

 size of the openings of the stomata varies, but also that transpiration, just as 

 evaporation from a surface of water, is dependent upon external conditions. Heat, 

 as well as the dryness and motion of the air, increases transpiration for purely 

 physical reasons ; while light, for physiological reasons, also promotes it. From 



