90 



BOTANY OF THE LIVING PLANT 



in the form of fluid water through the vessels, the dissolved substances 

 are conveyed upwards with it, and the stream is thus a means of 

 transit. This is in fact the real importance of the Transpiration- 

 Stream. 



Control by the Stomata. 



It has been seen that Transpiration varies in amount according to 

 the external conditions. Those which promote evaporation tend to 

 increase the stream. It is through the pores of the stomata that the 

 evaporation takes place. The loss which actually occurs can be 

 approximately measured by counterpoising on a balance a pot-plant, 



with its pot completely covered 



JK. 



by a rubber bag closed round 

 the stalk. Accurate weighings 

 taken at intervals will show 

 that the whole plant together 

 with its pot loses in weight. 

 The loss is approximately due 

 to Transpiration. Taking the 

 average loss per hour it is found 

 to be less by night than by day. 

 This may be partly explained by 

 the average temperature being 

 lower at night. But a more 

 important factor is the control 

 by the stomata themselves, which 

 are automatic mechanisms that 

 work in the following way. The 

 guard-cells contain chlorophyll, which is usually absent from the 

 ordinary epidermal cells. This produces under the incidence of sun- 

 light an increase of osmotically active substances such as sugars. As 

 the turgor of the guard-cells increases their curvature increases, 

 and the pore opens. In darkness, however, the osmotic substances are 

 gradually removed, and their turgor decreases. The guard-cells 

 then straighten themselves, and the pore closes (Fig. 67, A, B). That 

 their action depends upon the turgor of the cells is readily shown by 

 treating an open stoma with a 5 p.c. solution of common salt ; this 

 relieves the turgor, and the stoma may be seen to close, with ultimate 

 plasmolysis of its cells. The stomata are thus self-regulating organs, 

 usually open by day and closed by night. Their behaviour goes far 

 to account for the greater average transpiration of the plant by day 



Fig. 67. 



Stoma of Narcissus, in surface view. A, in the 



open ; B, in the closed state. ( x 250.) F. O. B. 



