56 SECRETION. 



50. For this reason gardeners sometimes make annular in- 

 cisions through the whole thickness of the bark around a branch 

 filled with fruit, so as to retain the nutritive juice, and augment 

 the size of the fruit. 



51. The greater part of the descending sap is found, as we 

 have before slated, in the bark ; but it appears that this liquid 

 also traverses the young layers of the albur'num, and it is by its 

 action that we explain the transformation of this albur'num into 

 perfect wood or dura'men. (Dura'men : Latin, hardening.) 



52. The descending sap appears to be chiefly composed of 

 water holding gum and some other substances in solution. It 

 must be regarded as the chief source from which the plant 

 derives the materials composing; 1st, the excreted products ; 2d, 

 the peculiar juices secreted in the different organs and designed 

 to remain in the interior of the plant ; 3d, the new tissues. We 

 shall now study these phenomena successively in order. 



OF SECRETIONS. 



53. Plants, as well as animals, form, in certain parts of their 

 bodies, peculiar liquids, which differ from the generally diffused 

 juices ; and it is to the process by which these peculiar liquids 

 are formed, as well as to the liquids themselves, that we give the 

 name of secretion* 



54. The matters secreted may be thrown out or expelled, or 

 they may be destined to remain in the interior of the plant, and 

 subserve the purposes of nutrition or some other function. 



55. The matters that plants excrete in this way are very vari- 

 ous. A great many plants produce in reservoirs, situate near 

 the external surface, volatile oils that evaporate through their 

 tissue and diffuse themselves through the air ; the odour of flowers 

 and also of certain leaves depends in a great measure upon this 

 exhalation ; and it is to an emanation of this kind that is due 



* Secretion : from the Latin, secer'nere, to separate. The process by 

 which organic structure is enabled to separate, from the fluids circulating 1 

 in it, other different fluids. The function of secretion is usually performed 

 by glands, and each gland secretes a peculiar fluid according to its struc- 

 ture ; for example, the liver secretes bile, that is, it separates from the blood 

 circulating in the liver, the materials which it forms into bile ; the sali'vary 

 glands secrete saliva, and the mammary glands in females, secrete milk, &c. 

 Now, bile, saliva, and milk, are also termed secretions 



50. How may the size of fruit be augmented ? 



51. Does the descending sap pass through any other part Jhaa the bark ? 



52. What are the chief uses of the descending sap ? 



53. What is meant by the term secretion ? 

 54 What becomes of the secretions ? 



55. Mention some of the various secretions of plants. 



