TRANSPIRATION 179 



of this phenomenon and tap the trees at just the right time to 

 secure the maximum flow of sap. Trees should be pruned at 

 such time as to avoid bleeding. 



Secretion is a very common phenomenon, but is always by 

 specialized cells which are frequently arranged to form definite 

 glands. It is more common than guttation or bleeding, the 

 variety of substances produced is much greater but the quan- 

 tities lost by the plant are much smaller. Some of these 

 secretions are very important in the life of the plant, but others 

 are apparently of no use and are considered waste products. 

 Among the most important of these secretions are the volatile 

 oils, alkaloids and glucosides (see Chapter XI). 



The glands vary in both structure and in the character of 

 their secretions. The simplest form of gland is a single epi- 

 dermal cell which may be on the same level or above or below 

 the level of its neighbors ; a more complicated type consists of a 

 mass of cells in a group or a filament ; or a group of cells lining 

 a simple or complex cavity. The glands in the skin of the orange 

 are very large and easily studied, but they are only one of the 

 many types of glands to be found in plants. The chemistry of 

 secretions is very imperfectly understood, but we know that some 

 glands secrete water, others sugar, others salts, others water 

 containing enzymes, others oil and still others the nectars. The 

 nectaries of flowers are highly developed glands. 



Water Transport. Since the water is taken in through the 

 roots of the plant and given out through the leaves and other 

 green parts, it is evident that it must pass through the stems. 

 With some few exceptions, we may say that the water of the 

 plant is always associated with substances in solution. The 

 movement of water in the living plant is very complex and as 



