2o8 SECRETION AND EXCRETION 



tion seems to be a method of accumulating it for use at some 

 future time. 



The Process of Secretion. — Secretion is evidently a vital 

 process, that is, it is carried on by the living protoplasm. The 

 substances may be formed directly from foods, or from the 

 disintegration of the cell-wall, or from the decomposition of 

 the protoplasm itself. It is possible that sometimes all of these 

 methods are employed by a single cell or gland. The distinctive 

 behavior of the cytoplasm of some secreting cells is evidence 

 that it produces the secretion by self-decomposition; for, pre- 

 paratory to secretion, the cytoplasm becomes relatively very 

 dense and granular, and as secretion sets in this density dimin- 

 ishes, until, when secretion stops, the cytoplasm has become 

 very much depleted. In some Rutaceae the cytoplasm and 

 nucleus of secreting cells disappear altogether. 



In some instances the evidence is very clear that the secretion 

 has been formed from the substance of the cell-wall, where the 



secretion appears just 

 beneath the cuticle, and 

 accumulating, pushes 

 the cuticle off from the 

 wall, as seen in Fig. ii8. 



Fig. ,.8.-Glands from the leaf of Ribes nigrum. SccrctionS of mudlage 



.(4, young stage in the development of the gland where and of ethereal oils and 

 the cuticle is already being pushed up by the secre- . ., . ■ i_* 



tion, «'. 5, completegland;fe, secretingcells;/i, cavity reSmS taKC plaCC lU tnlS 



between the secreting cells and cuticle occupied by way Of rOllTSe thp 



the secretion. (After Haberlandt.) -^ ' 



secretions do not neces- 

 sarily come entirely from the substance of the wall, for it is 

 possible that the protoplast makes a part of the secretion directly 

 without first repairing the wall preparatory to decomposing it. 



The Excretion of Liquid Water. — On summer nights 

 "dew" hangs in droplets at the tips and along the edges of 

 leaves of grass and many other kinds of plants. It was long 

 supposed that in all cases these droplets were real dew formed 

 from condensation from the atmosphere, but that this is by no 

 means always the case is shown by chemical analysis of the 



