THE MATERIAL OUTGO OF PLANTS 327 



tion and entrance of the gases absolutely indispensable to life. To be 

 sure, the outer walls of the surface cells are relatively dry, especially in 

 plants of dry regions, where water loss is to be reduced to a minimum. 

 Of the total water lost scarcely more than 20 per cent, and as little as 3 

 per cent, escapes through the epidermis. This evaporation is sometimes 

 distinguished as cuticular transpiration. The remaining 80-97 P er cent 

 diffuses through the stomata and constitutes stomatal transpiration. The 

 efficiency of this arrangement in reducing transpiration and yet admit- 

 ting gases freely is more obvious when one observes that the actual 

 evaporation surface — i.e. of the cells bounding the intercellular spaces 

 — is several times that of the leaf itself. 



The place of maximum cuticular evaporation has been shown in some leaves to 

 be that part of the outer wall of the epidermis where the side walls abut. In these 

 cases water of imbibition is more abundant there than elsewhere. 



It is impossible to determine the actual surface exposed in the very irregular air 

 passages. If a leaf 1 mm. thick had an epidermis 0.1 mm. thick of coherent cubical 

 cells on each face, and if the remaining cells were spheres each 0.1 mm. in diameter, 

 tangent to each other, the internal surface would be about fifteen times the area of 

 the corresponding outer faces of the leaf. This, of course, does not pretend to pic- 

 ture the actual state of affairs; but it will give an idea of the relative magnitudes 

 involved. 



Stomata. — The guard cells of the stomata are different from the rest 

 of the epidermal cells in form, in the peculiar unequal thickening of 

 their walls, and generally in the possession of chloroplasts. These 

 characters and the position of the guard cells with reference to the ad- 

 jacent subsidiary cells determine simultaneous differences in turgor 

 and make them behave differently from the others. In general when 

 turgid, they become arcuate, and when flaccid, they straighten. The 

 mechanics of these movements differs considerably with differences of 

 form, structure, and position, and in none of the several types that have 

 been distinguished is it fully understood. The chloroplasts are supposed, 

 but on no very good grounds, to impart power to make osmotically 

 active substances that do not exist in adjacent cells (or are present in 

 smaller amount), so that these cells may be more turgid than the others 

 with the same water supply. The longitudinal thickenings are elastic 

 and are supposed to straighten the cells when they become flaccid. 

 The auxiliary cells are supposed to offer proper bracing for the guard 

 cells so that turgor will arch them. 



Regulation by stomata. — Naturally the guard cells are most likely 

 to be turgid when the water supply is good; then the opening of the slit 



