218 STEUCTUEAL BOTANY 



through the stoma into the air-chamber, and is thence 

 absorbed by the mesophyll-cells surrounding it, the 

 walls of which, being of cellulose, do not hinder the 

 passage of the gas. The carbon dioxide thus absorbed is 

 at once dissolved in the cell-sap, and it is this dissolved 

 gas which is decomposed by the chlorophyll-corpuscles. 



The extremely small size of the stomatal pores, 

 through which the gases have to pass, may seem a diffi- 

 culty, but it has recently been proved that the rate at 

 which a gas will diffuse through an opening varies, not 

 with the area of the opening, but with its diameter. 

 Hence a large number of minute pores is far more 

 effective as a means of diffusion than a smaller number 

 of larger pores, and the conditions in the epidermis of 

 a leaf prove to be specially favourable for a relatively 

 rapid passage of gases. 



The mineral substances taken up by the root are 

 absorbed by the root-hairs, where these are present, 

 and must be taken up in solution. Yet we know 

 that many plants nourish well in a comparatively dry 

 soil. In such a soil, however, each particle of earth 

 is surrounded by a film of water, which adheres to it 

 firmly. The root-hairs attach themselves to such 

 particles, and are thus enabled to absorb a part of the 

 watery film which surrounds them (see Fig. 29, p. 77). 

 The root-hairs, however, are not dependent entirely on 

 salts which are ready dissolved in the water of the 

 soil. They are able to dissolve other compounds for 

 themselves. Calcium phosphate, for example, is only 

 soluble to a very small extent in water. The root- 

 hairs, however, excrete an acid by which minute 

 quantities of such salts are brought into solution, and 

 can then be absorbed. This solvent action of the 

 root-hairs can be proved by growing plants in a thin 

 layer of earth covering a slab of polished marble. 

 After some time, an exact print of the course of tha 



