230 BOTANY PAET i 



salts and to reduce their temperature, make little use of such pro- 

 tective coverings. On the contrary, it has been seen (p. 168) that 

 they are provided, besides the adaptations to regulate the transpiration, 

 with special contrivances for promoting evaporation. Their great surface 

 extension may be specially mentioned. 



Transpiration is not, however, limited to the cells which are directly 

 in contact with the atmosphere ; an enormous number of internal cells 

 can get rid of water vapour when they abut on intercellular spaces. 

 The air-filled intercellular spaces would clearly become after a short 

 time completely saturated with water vapour were they completely 

 closed. Communications exist, however, as we have seen, between 

 the atmosphere and the intercellular spaces, the most important 

 being the stomata (p. 51). The aqueous vapour can escape by these, 

 and thus the condition of saturation of the air in the intercellular 

 spaces is not complete. The water vapour escaping from the stomata 

 is readily recognised by means of cobalt paper. If pieces of this are 

 laid at the same time on the upper and lower surface of a leaf that 

 has stomata only on the lower side, a change of colour will take place 

 in the cobalt paper on this side, while no appreciable giving off of 

 water will be shown for the upper side. 



It is usual to distinguish stomatal and cuticular transpiration, and 

 we may thus say that only the stomatal transpiration is of importance in 

 the typical land plant. In plants inhabiting damp localities the cuti- 

 cular transpiration becomes considerable. Though the openings of the 

 stomata are extremely small (the breadth of the pore being 0*007 mm. 

 and less) so that neither dust nor water can pass through them into the 

 plant, they are usually present in such enormous numbers and so suitably 

 distributed that their united action compensates for their minuteness. 

 When it is taken into consideration that, as NOLL has shown, a medium- 

 sized Cabbage leaf (Brassica oleracea) is provided with about eleven 

 million, and a Sunflower leaf with about thirteen million stomata, 

 it is possible to estimate how greatly evaporation must be promoted 

 by diffusion through these fine sieve-like perforations of the epidermis 

 and of the cuticular membrane which allows practically no water to 

 pass. BROWN and ESCOMBE have shown that the movement of 

 diffusion through this perforated membrane is as rapid as if no cuticle 

 were present. If this is correct the question presents itself, why 

 the plant has constructed such a complicated apparatus instead of 

 allowing free transpiration from unprotected cells. The explanation 

 lies in the fact that the stomata not merely facilitate transpiration, 

 but can stop it; they serve to REGULATE the transpiration, which 

 a cuticle cannot do. The width of the pore of the stoma can be 

 altered by changes in the guard cells. When the pore is fully opened 

 transpiration is maximal, and when it is completely closed transpira- 

 tion sinks to zero. Since the opening and closing of the pore take 

 place in accordance with the needs of the plant, the stomata are 



