)0 



BOTANY OF THE LIVING PLANT 



.n tlic form of fluid water through the vessels, the dissolved substanees 

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

 :ransit. This is in fact the real importance of the Transpiration- 

 Stream. 



COXTROL BY TUE StOIIATA. 



It has been seen that Transpiration varies in amount according to 

 :he external conditions. Those which promote evaporation tend to 

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

 jvaporation takes place. The loss which actually occurs can be 

 ipproximately measured bv counterpoising on a balance a pot-plant, 



with its pot completely covered 



A.. 



B 



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 

 miportant factor is the control 

 bv the stomata themselves, which 

 are automatic mechanisms that 

 work in the following way. The 

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

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

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

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

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

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

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

 heir action depends upon the turgor of the cells is readilv shown bv 

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

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

 lasmolysis of its cells. The stomata arc thus self-regulating organs, 

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

 3 account for the greater average transpiration of the plant bv day 



I'IG. 67. 



tuiiui uf Xurcissiis, in surface view. A, in tJie 

 nprn ; i', ill the closed state. (K250.) F. O. B. 



