ii6 THE PHYSIOLOGY OF PLANTS 



become turgid when the whole leaf is turgid, that is, when the 

 roots supply the leaf with a large amount of water, and when 

 there might arise some danger of a superfluity of water in the 

 leaf tissues. In such cases, the plant protects itself against 

 danger by enlarging the passages through which the water 

 vapour can escape. On the other hand, the tissues in the 

 inside of the leaf are protected from too great a loss of 

 water through evaporation ; for as soon as the guard cells 

 lose water and become less turgid, they close the aperture 

 which existed between thera. Thus the plant regulates 

 the amount of its transpiration according to its needs and 

 requirements. 



In many plants such safety-valves are still more largely 

 developed. The tips and teeth of some leaves have very large 

 stomata, arranged singly or in groups, and their guard cells 

 are either rigid or sometimes quite destroyed. They lie 

 immediately above a specially formed very thin-walled paren- 

 chymatous tissue (epithem), which reaches up to the epidermis. 

 The vascular bundles of the leaf terminate in this epithem 

 by a number of lignified cells or tracheids, often arranged after 

 the manner of a brush. Now if the tissues of a plant contain 

 an excessive amount of water, it is given off in these places 

 in the form of liquid drops of water. Here, therefore, the 

 stomata are water stomata or water glands. 



This explains the phenomenon, which is well known to all 

 gardeners, that in cool nights the tips and teeth of the leaves 

 of some plants {Primula sinensis, Calla cethiopica, &c.) are 

 quite regularly studded with small drops of water, and new 

 drops will make their appearance as soon as the first are 

 removed. 



Just like the leaves, the green herbaceous stems are also 

 covered by an epidermis perforated by stomata, so that the 

 cortical tissues can be aerated. When the stem grows older, 

 and the simple epidermis is replaced by a more resistant layer 

 of cork, we find that below the stomata those more complicated 

 respiratory organs will be formed which we have already dis- 

 cussed — namely, the lenticels. 



The description of the epidermis would be incomplete if we 

 omitted to mention its excrescences or triehomes. We under- 



