196 



BOTANY 



PAET I 



The stomata are mainly present on the leaves, which are thus to 

 be regarded as organs of transpiration (and of assimilation, p. 216). 

 The amount of water evaporated from the leaf surfaces in the 

 performance of their vital functions is surprising. 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. 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. It has been calculated that during the period of vegetation 



FiQ. 181. — Stoma of Helleborus sp. in transverse section. The darker lines sliow the shape assumed 

 by the guard-cells when the stom.a is ojien, the lighter lines when the stoma is closed. (After 

 ScHWENDENER.) The cavities of the guard-cells with the stoma closed are shaded, and are 

 distinctly smaller than when the stoma is open. 



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. 



It is evident from these and similar experiments that more Mater 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 j^eculiarities 

 (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 only the 

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

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

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

 jjh3'sical reasons ; while light, for physiological reasons, also promotes it. From 

 both physical and physiological causes, transpiration is more vigorous during the 

 day than night. Plants like Impatiens -parviflora, which droop on warm days, 

 become fresh again at the first a})iiroaeh of night. 



Special Contrivances for regulating the Water-supply (''). — Almost all the 

 higher plants possess, in the power to close their stomata, a sjiecial means of 



