How Substances are Transported and Removed 217 



to other matters. Presently, however, somebody, working per- 

 haps in a quite different field, chances upon some new method 

 that happens to be applicable to this subject, to which students 

 then turn once more, and make another long step in advance. 

 The very fact that all knowledge thus grows by appreciable 

 stages makes it all the more interesting to follow; and the watch- 

 ing for such new knowledge, and the grasping it when it appears, 

 constitute the principal charm of the scientific life. 



There remains but one other point in connection with the 

 transfer of water. The current must supply not only the tran- 

 spiration loss, but all the working needs, chemical, osmotic and 

 other, of the various tissues besides. This matter, however, is 

 simple, for all kinds of ducts possess plenty of thin places through 

 which the water can pass outward, after which, by imbibition 

 and osmosis, it gradually penetrates from cell to cell throughout 

 all of the tissues that need it. And with the water in this way go 

 the various minerals in solution, which explains their transporta- 

 tion, as well as their absorption, by the plant. 



From the transport of water and minerals we turn to that of 

 the various food-substances made in the plant, a subject known 

 in plant physiology as translocation. The subject is comparatively 

 simple. In the first place such substances travel invariably in 

 solution; and substances which are not soluble in water never 

 move from their places of formation. The very physical nature 

 of some substances, e. g. the sugars, makes them naturally soluble, 

 but others, viz. starches, oils, cellulose, and most proteins, are for 

 the same reason insoluble. In such cases solubility is obtained, 

 for purposes of translocation, by their conversion (or hydrolysis) 

 into closely-related substances which are soluble, thus starch 

 and cellulose into sugar, oils into fatty acids, insoluble proteins 

 into peptones. These changes are effected by those remarkable 

 substances called enzymes, whose method of action we have con- 

 sidered in the chapter on Metabolism. The enzymes are widely 

 scattered through plants, and some of them are identical with the 



