CHAPTER XII 

 SECRETION AND EXCRETION 



Nature of Secretions and Excretions. — Secretions and 

 excretions are distinguished from the reserve food told about 

 in the last chapter by their evident uselessness in supplying 

 materials and energy for grovi^th, repair, etc. They there- 

 fore, for the most part, remain practically unchanged in special 

 cells or tissues, or are eliminated from the cells by excretion 

 at the surface of the plant or into intercellular spaces. Some- 

 times they have a biological function, as in the case of nectar, 

 and sometimes a physiological function, as in the case of enzymes, 

 and organic acids excreted by the roots. 



By far the larger number of plant secretions belong to the 

 class of ethereal oils and resins. These occur together, the 

 resins dissolved in the oils and forming oleo-resins. When 

 the oil evaporates, as it normally does in time, and can quickly 

 be made to do on heating, the resins are left behind as a solid 

 residue. 



The amount of resin present varies greatly, from the merest 

 traces in many epidermal glands to more than 70 per cent, in 

 some of the Coniferae. 



The fragrance of these secretions is due to volatile substances 

 entering into the composition of, and even forming a very large 

 part of, the oils, as, for instance, eugenol in clove oil, safrol in 

 oil of sassafras, and cinnamon aldehyde in oils of cinnamon 

 and cassia. 



Turpentine, produced chiefly by the Coniferae, is the most 

 abundant of the oleo-resin secretions. On distillation this 

 yields oil of turpentine of commerce and resins varying in char- 

 acter with the different genera producing them. 



No sharp line can be drawn between secretions and excre- 

 tions as these words have come to be used in physiological litera- 



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