570 PHYSIOLOGY. 



Circumstances regulating Transpiration. The amount of transpiration 

 depends on the amount absorbed, the quantity of water in the tissues, the 

 age of the plant, the amount of surface exposed (Asa Gray calculated that 

 a moderate sized Elm-tree bore seven millions of idRves, the total surface 

 being equal to 5 acres), the nature of the epidermis, the texture of the leaf, 

 &c. : thus it is usually greatest from the lower surfaces of leaves, which are 

 provided with the greatest number of stoinata. External conditions, such 

 as the degree of moisture in or the temperature of the air, exert great 

 influence on transpiration ; the drier and hotter the atmosphere, the greater 

 the transpiration ; but, according to McNab, plants exposed to the sun 

 transpire most in a moist atmosphere, while in the shade transpiration 

 ceases when the atmosphere is loaded with watery vapour. Light also 

 has great effect on the quantity evaporated. McNab's experiments, 

 however, show that the rate of ascent of the watery sap is not checked 

 by placing the branch in darkness for a short time. M. Wiesner shows 

 that part of the light which traverses the chlorophyll is transformed 

 into heat, as a consequence of which there results a rise of tem- 

 perature, and an increased tension in the watery vapour in the inter- 

 cellular passages occurs. The excess of vapour escapes by means of the 

 stoinata. A plant may, as shown \)j McNab and Deherain, transpire in 

 a saturated atmosphere, but only under the influence of light. 



M. Wie^ner has studied transpiration in three different ways: 1, 

 by comparing that of green plants with that of blanched ones ; 2, 

 by exposing the plants in the solar spectrum ; and 3, by placing them 

 behind solutions of chlorophyll. By these different ways, he has arrived 

 at the same results, viz. : that the presence of chlorophyll markedly 

 increases the action of the light on transpiration; that it is the rays 

 which correspond to the absorption-band of the chlorophyllian spec- 

 trum, and not the most luminous rays, which excite transpiration ; and 

 lastly, that the rays which have traversed a solution of chlorophyll have 

 only a slight influence on transpiration. Other colouring-matters, as xan- 

 thophyll, for instance, may act in the same way as chlorophyll, but to a less 

 extent. The opening of the stomata may accelerate the transpiration ; but 

 the very marked transpiration of young Maize plants, the stomata of which 

 were closed, and the slight transpiration of a Hartwegia comosa, the 

 stomata of which were widely open in obscurity, suffice to show that this 

 cannot be the principal cause of the transpiration in the light. In a very 

 positive manner, but in a less degree than in the case of the luminous rays, 

 the obscure calorific rays act. As to the chemical rays beyond the violet 

 their action is null or very slight. Whatever may be the nature of the 

 rays, they always act in raising the temperature of the tissues. 



In spring, before the expansion of the buds, absorption is necessarily 

 greater than transpiration ; the water in such a case is stored in the stem, 

 wheie it is made available for the expanding buds and growing tissues 

 generally. In summer the transpiration is greater than the absorption ; 

 and then the leaves depend for their supply on the stores in the stem, 

 or, failing that, they wither. Even in winter, provided the stem be not 

 absolutely frozen, there is a motion of the juices, dependent to a great 

 extent on the temperature of the soil, which is always in that season higher 

 than that of the air, and it increases in amount from the surface down- 

 wards. 



