2OO 



SECRETION AND EXCRETION 



ture. So long as the substance is in the cell that formed it we 

 are apt to think of it as a secretion, but as an excretion when 

 it is eliminated from the cell into intercellular spaces or at the 

 exterior. The significance of these terms will be better appre- 

 hended by means of their particular application as we proceed. 

 Secreting Cells and Glands in General. Probably all 

 living cells secrete digestive and oxidative enzymes, and all 

 cells secrete their cell- walls; but in certain numerous families 

 of plants we find single cells or groups of cells called glands 

 that carry on secretion as their special function. The secret- 

 ing cells are sometimes descended from the protoderm, and 

 therefore belong morphologically to the epidermis, and some- 

 times they are descendants of the fundamental meristem and 

 may occur in the cortex, pericycle, medullary rays and pith. 



FIG. 108. Glandular hair from the petiole of Pelargonium zonale. e, secretion from 

 the globular gland on which it rests; B, portion of across section through a nectariferous 

 bract of Vicia sepium; n, nectar-secreting cells. (After Haberlandt.) 



Less frequently we find them descended from the procambium 

 and belonging to the phloem or xylem parenchyma, as in the 

 case of some of the resin-secreting cells of Conifers. 



There are three kinds of glands in regard to their location 

 and form, namely, the superficial type which, descended from 

 the protoderm, is borne at the outer surface and may rise above 

 it in the form of hairs or scales; the interior globular type con- 



