78 , THE PHYSIOLOGY OF PLANTS 



this destruction of various substances may become very disas- 

 trous to tlie plant, as we shall have occasion to see in dealing 

 with the " over-watering " of pot-plants. 



If we are to take any account of the favourable conditions 

 for aeration of plants, we must first of all study the contrivances 

 which exist in plants for the interchange of gases. 



In the first place, we must mention the breathing-pores 

 (stomata) which occur in all green organs of any considerable 

 size. These pores, which occur especially on the under-surface, 

 sometimes on both sides of leaves, belong to the external layer 

 of cells (epidermis), and consist of two semilunar or kidney- 

 ehaped cells, facing each other with their concave sides, and 

 attached firmly together at their pointed ends. They contain 

 chlorophyll grains, which are generally absent from other 

 epidermal cells. 



As the pointed ends of these sickle-shaped cells meet, tHey 

 leave an elliptical space between them, and this represents the 

 chimney or passage by which the air has access to the interior 

 of the plant. Every such passage opens internally into a 

 large cavity in the tissues of the leaf which lies immediately 

 beneath the epidermis, and is called the respiratory cavity. 

 From this cavity a large number of small irregular passages 

 run in all directions, formed by small spaces (intercellular 

 spaces) which exist between the several green cells of the 

 leaf. The external air can pass, therefore, into the respiratory 

 cavity, and from this latter through the intercellular spaces to 

 every single cell of the leaf. Gases can therefore pass directly 

 into every cell or out of every cell. In submerged portions 

 of plants the cells derive the necessary oxygen from the oxygen 

 contained in the water. 



In very delicate green structures, and in those portions of 

 plants which are devoid of chlorophyll, the stomata are absent, 

 while in some leaves they may number 1 8o,000 per sq. inch. 

 In tissues which are devoid of stomata the interchange of 

 gases is effected by the entire surface of the organ or by other 

 structures. 



In older stems and roots these structures are the so-called 

 lentlcels. 



These are small structures penetrating into the cortex, and 



