RETURN OF THE SAP. 35 



now takes place, and new modifications are in con- 

 sequence produced, which, from the intricacy of 

 the chemical processes thus conducted in the inner 

 recesses of vegetable organization, we are utterly 

 baffled in our attempts to follow. All that we are 

 permitted to see are the general results, namely, 

 the gradual deposition of the materials of the future 

 alburnum and liber. These materials are first depo- 

 sited in the form of a layer of glutinous substance, 

 termed the Cambium ; a substance which appears 

 to consist of the solid portion of the sap, precipi- 

 tated from it by the separation of the greater part 

 of the water that held it in solution.* The cam- 

 bium becomes in process of time more and more 

 consolidated, and acquires the organization proper 

 to the plant of which it now forms an integrant 

 part : it constitutes two layers ; the one, belonging 

 to the wood, being the alburnum ; the other, be- 

 longing to the bark, being the liber. 



The alburnum and the liber, which have been 

 thus constructed, perform an important part in 

 inducing ulterior changes on the nutrient materials 

 which the returning sap continues to supply. Their 

 cells absorb the gummy substance from the sur- 

 rounding fluid, and by their vital powers effect a 

 still further elaboration in its composition ; con- 

 verting it either into starch, or sugar, or lignin, 

 according to the mode in which its constituent 



* A substance, containing a large proportion of nitrogen, and 

 yielding ammonia on distillation, is deposited on tlie walls of the 

 cellules of the roots, and especially of the spongioles. This sub- 

 stance abounds in the cambium, prior to its becoming oxygenized ; 

 and it exists in all the young parts of plants, during their develope- 

 ment. See Payen, Comptes Rendus, 1838, p. 132. 



