CIRCULATION OF THE SAP. 165 



of its oxygen. Thus there is constantly going on 

 between the cells a process like that which may be 

 observed if a membraneous bag containing a thick 

 fluid should be immersed in water, — the thick 

 becomes thinner by the absorption from the latter, 

 or endosmose, while the water derives a small 

 quantity of the former. Hofmeister discovered that 

 this was the cause which produced the circulation 

 of the sap from the root to the leaf, and that the 

 little white fibres, styled spongioles by old writers, 

 which were emitted during each season of growth, 

 were formed of cells not covered by the thick skin 

 or epidermis of the older roots, and that they absorb 

 the thin fluid of the soil, which contains pabulum 

 in solution. Thus a regular grade of elaboration, 

 or thickness of the sap, is established from the leaf 

 to the roots. These absorbing cells, after having 

 performed their work for one season, become cov- 

 ered with the epidermis, and cease to provide for the 

 growth of the tree. This should be borne in mind 

 by the cultivator when he applies his manure to the 

 ground about his trees. The feeding roots are at a 

 considerable distance from the trunk of the tree, 

 and if applied near it, scarcely any benefit will 

 accrue. 



Roots have been said to excrete substances ab- 

 sorbed, which were unnecessary or injurious. This 

 was the belief of De Candolle, and others. It was 

 formed from the fact that a plant will not succeed 



