SECT. II PHYSIOLOGY 195 



transpiration, with special contrivances for promoting evaporation. 

 Their great surface extension, by means of which a relatively large 

 number of cells reach the surface, 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 an intercellular space. 

 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. 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 recog- 

 nised 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 cuticular transpiration becomes considerable. Instead 

 of cuticular and stomatal transpiration the expressions external and 

 internal transpiration may be used. Though the openings of the stomata 

 (p. 114) are extremely small (the breadth of the pore being 0"0007 mm. 

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

 plant, they are usually present in such enormous numbers (p. 114) 

 and so suitably distributed that their united action compensates for 

 their minuteness. When it is taken into consideration that a medium- 

 sized Cabbage leaf (Brassica oleraxea) 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 

 (cf. p. 213). 



The stomata also afford plants a means of regulatino evapora- 

 tion. The pores, which are the mouths of intercellular spaces, are 

 surrounded by two GUARD-CELLS (p. 113). As the term guard-cell 

 suggests, these cells have the power of closing the pore. The 



CLOSING AND OPENING OF THE STOMATA ARE ACCOMPLISHED BY 

 CHANGES IN THE TURGIDITY OF THE GUARD-CELLS OR OF THE AD- 

 JOINING EPIDERMAL CELLS. In consequence of their peculiar wall 

 thickenings, elasticity, and lateral attacbment, a change of turgidity 

 affects the size and shape of the guard-cells in such a way that, on 

 diminished turgidity, they become flatter and close the air-passage, 

 while an increase of turgidity has the contrary effect and opens the 

 pore (Fig. 81). In many plants the so-called subsidiary cells (p. 114) 

 participate in various ways and degrees in these processes. 



