PHYSIOLOGY 



191 



the soil, the organic building material of the plant-body is produced. 

 For similar reasons, it is in the leaves that the broad expansions of tissue 

 for the special promotion 

 of transpiration are found. 

 The amount of water actu- 

 ally evaporated from the 

 leaf surfaces in the per- 

 formance of their vital 

 functions is almost in- 

 credible. For instance, a 

 strong Sunflower plant, of 

 about the height of a man, 

 evaporates in a warm day 

 over a litre of water. It 

 has been estimated that 

 an acre of cabbage plants 

 will give off two million 

 litres of water in four 

 months, and an acre of 

 hops three to four millions. 

 The quantity of water daily 

 required to maintain the 

 water-supply of a single 

 large tree, amounts to 

 many litres. The water 

 evaporated in the five 

 months from June to 

 November from an Oak 

 standing perfectly free and 

 apart, and having about 

 700,000 leaves, has been 

 estimated at 111,225 kilo- 

 grams. According to Dietrich, for every gramme of dry, solid matter 

 produced, there is, on the average, 250-400 grams of water evaporated. 



Fig. 178.— Course of the vascular bundles (venation) in a 

 leaf of Crataegus. (Prom a photograph ; natural size.) 



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 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 concen- 

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

 of the absorbing substance to estimate the amount of water given off by evapora- 

 tion. 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 lowering 

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



