192 



BOTANY 



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 can thus also be utilised to deter- 

 mine any variation in the size of the open- 

 ings of the stomata. 



It is evident from these and similar ex- 

 periments that more water is evaporated in 

 a given time from some plants than from 

 others. These variations are due to differ- 

 ences 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 only the 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 physio- 

 logical reasons, also promotes it. 



From both physical and physiological 

 causes, transpiration is much more vigorous 

 during the day than night. Plants like 

 Invpatiens parviflora, which droop on warm 

 days, become fresh again at the first approach 

 of night. 



Suction in Tkanspikixg Shoots. — A 

 shoot, the cut end of which is placed in 

 water, shows by remaining fresh that it 

 must be able to draw up water to its ex- 

 treme tips. The force of suction exerted 

 by such a transpiring foliage shoot may be 

 demonstrated, by fitting the cut end in a 

 long glass tube filled with water in such a 

 manner that it shall be air-tight. Thus 

 up the tube by the suction exerted by arranged, the shoot will be able to sustain 

 the transpiring shoot. (From Detmeb's an ^ ra ; se a column of water 2 metres high. 

 Physiol. Fran.) Tf th(J lower end Qf ^ tube ^ inserted 



in mercury, as shown in the adjoining figure (Fig. 179), it will be found that 

 even the heavy mercury will be lifted by the transpiring shoot to a consider- 



Fig. 179. — Suction of a transpiring shoot. 

 The leafy shoot is fitted so that it is air- 

 tight in a glass tube filled with water and 

 with the lower end immersed in a vessel 

 of mercury. The mercury is drawn 



