246 



BOTANY 



PART I 



soluble in water than is oxygen, it will be evident that it will 

 pass more rapidly through a cell wall saturated with water than 

 oxygen will. In all probability this holds for the cuticle as well. 

 Since, however, the partial pressure of the oxygen in the air is 

 relatively considerable, while that of carbon dioxide is very slight, 

 oxygen can pass in sufficient quantity through the cuticle, but carbon 

 dioxide cannot ; on this account we find that all organs which only 

 require to absorb oxygen are unprovided with stomata, while organs 

 which absorb carbon dioxide always have stomata. 



In the soil as well as in the air, plants, as a rule, find so much oxygen that this 

 gas is able to pass through the epidermis. Organs which live in swampy soil 



which is poor in oxygen form an exception to this. 

 In marsh plants, which stand partly in the air, the 

 large intercellular spaces form connecting canals 

 through which the atmospheric oxygen without 

 being completely used up can reach the organs grow- 

 ing deep in the swampy soil and cut off from other 

 supplies of oxygen. In some cases (especially in 

 Palms and Mangroves) the need of a supply of oxygen 

 to such roots is met by specialised roots (PNETJMATO- 

 PHORES) which project vertically from the muddy 

 soil (Fig. 188), and absorb oxygen from the air. 

 The efficiency of the stomata in gaseous exchange 

 varies with the width to 

 which the pores are open. 

 The closure of the pores of 

 the stomata, which may be 

 brought about in maintain- 

 ing a sufficient supply of 

 water, not only arrests 

 FIG. 246.-Diagram of an experiment to demonstrate the transpiration, but also pre- 



inovement of air through the stomata. vents the entrance of C0 2 



into the plant. 



It has been seen in considering the giving off of water vapour that the stomata 

 in spite of their small size facilitate diffusion on account of their enormous 

 numbers and their distribution. This also applies to the absorption of carbon 

 dioxide. Thus, for example, a square metre of the surface of a Catalpa leaf absorbs 

 about two-thirds the amount of carbonic acid gas taken up in an equal time by the 

 same area of potash solution freely exposed to the air. 



The Movement of Gases from cell to cell and their interchange 

 between the cells and the intercellular spaces takes place by diffusion. 

 In the intercellular spaces movements in mass due to pressure are 

 concerned. Unequal pressure is set up by the warming or cooling 

 of the air in the intercellular spaces, or by movement of the part of 

 the plant leading to changes of shape. The intercellular spaces form 

 a highly-branched system of cavities communicating with one another 

 and with the atmosphere. The communication with the outside is 

 effected in the first instance by the stomata, and also by the lenticels 



