251 



some trees, or in certain parts of trees, it may be wholly wanting, while 

 in others it may occur in abundance. It has not been proven, however, 

 that soda is entirely wanting in any entire tree or plant grown on natural 

 soil. The general conclusion is, then, that the quantity of soda present in 

 a tree is an extremely variable one, and though generally present in some 

 proportion, yet in some parts of a tree it has not been found present in 

 weighable quantities. 



An important question now arises : Can soda take the place of potash ? 

 The result of the investigations of Halm-Horstmar, and more recently of 

 Knop and Schreber, have demonstrated that it cannot entirely do 

 this, since potash is absolutely essential to the growth of the tree. 

 Cameron concludes that soda can partially replace potash, and this 

 appears to be indicated by many facts. It may be, however, that the soda 

 which often appears to replace the potash is accidental, and that the re- 

 placed potash was present in the tree in an excess of that really needed for 

 its growth. The amount of either absorbed would depend on the nature 

 of the soil. 



Potash is of the next importance to the vegetable world. The organic 

 acids require alkalies and alkaline earths to form the salts which exist in 

 the tree, and in most cases it would be impossible for these acids to be formed 

 were it not for the presence of these bases. There is every reason to believe 

 that the alkalies are peculiarly connected with the formation of carbohy- 

 drates, and that an increased assimilation of alkalies is co-ordinate with 

 the increased formation of carbohydrates. 



Lime is of the next importance. Its great abundance in nature is a 

 guarantee of its presence in an available form for the tree. 



Of the remaining ingredients, iron, is perhaps, of the greatest import- 

 ance. It is abundantly proven that its presence is necessary to the devel- 

 opment of the tree or plant. It is usually found in the greatest abundance 

 in the bark ; much of it is accidental and not necessary to the develop- 

 ment of the tree. 



CI is never totally absent from a tree, but if necessary to its develop- 

 ment, only a small quantity is needed. Its absence in many reported ana- 

 lyses is due, without doubt, to the fact that it is easily driven off from the 

 ash when it is at the temperature necessary for the combustion of the 

 wood. 



Silica is not indispensable to the growth of trees but analyses show that 

 it is always present in the ash of trees grown on natural soils. It is usually 



