80 BOTANY. 



decomposition, thus allowing their assimilation. They are 

 commonly called the "ash" of plants, and are often erro- 

 neously regarded as consisting of unassimilated matter. 

 That they enter into the vital activities of "the plant has 

 been shown by the experiment of withholding them, with 

 the result that the plant so treated always languishes or 

 dies. 



140. Further Chemical Changes. — Even after the 

 various substances which constitute plant-food have become 

 assimilated they undergo many chemical changes. Every 

 living tissue, and perhaps every living cell, is the seat of 

 chemical changes in assimilated matter, whose results have 

 in many cases been made out by chemists who have made 

 numerous analyses, but in no case are the details of these 

 chemical changes certainly known. We know that in 

 many of these operations oxygen is absorbed by the active 

 cells, and that as one result of their activity they excrete 

 carbon dioxide. These after-changes of assimilated matter 

 have been known in physiology as metastasis or metabolism. 



141. Digestion and Use of Starch. — Among the most 

 important of the subsequent chemical changes are those 

 which render the starch in the chloroplasts soluble, allow- 

 ing it to diffuse to other parts of the plant with great free- 

 dom. The nature of these changes appears to vary some- 

 what in different plants, but they consist essentially in the 

 change of the insoluble starch into a chemically similar but 

 soluble substance. Glucose (C,H„0,), inulin (C,H,„OJ, 

 and cane-sugar (C„H„0„) are the more common of the 

 soluble substances so formed, and one or other of these 

 may frequently be detected in the adjacent cells after the 

 disappearance of the starch from the chlorophyll. 



148, These diffusing carbohydrates are imbibed by the 



