THE LIFE HISTORY OF A TREE 9 



the expense of the sugar begins. As the starch is insoluble 

 the action of photosynthesis can continue unchecked, seeing 

 that the concentration of the sugar will not rise beyond the 

 critical point. Sugars, as they arise, are also continually 

 passing away from the leaf to other parts of the tree, there 

 to be used up, along with other materials, in the formation of 

 new living substance, wood and other materials, as well as to 

 supply the chemical energy needed for vital processes. The 

 sugar thus translocated may also accumulate in other parts of 

 the tree, and, by stimulating the production of starch in those 

 regions, e.g. in the medullary rays or silver grain of the wood, 

 provide reserve stores of carbohydrate to be drawn on as 

 occasion may arise. When the photosynthetic activity of the 

 leaf ceases, e.g. at night time, the starch formerly produced 

 in the leaves is reconverted, by special ferments', into sugar, 

 which thus continues to replace that which is being still with- 

 drawn from the leaves till all the starch may be finally re- 

 moved from them. This explains why it is that towards the 

 end of a summer's day the leaves may be full of starch, but 

 next morning may contain little or none. 



It must be clearly understood that starch, as such, is of no 

 direct use as a food it must first be changed into a soluble 

 sugar before it can be employed in the life processes of the 

 plant. The sugar is the utilizable floating capital, the starch 

 the bank reserve, of the plant. 



The water from the roots passes upwards to the leaves in 

 the outer layers of the wood, and the sugary sap travels from 

 the leaves to the parts where it is required in the inner layers 

 of the bark. Hence if a tree is * girdled ' by having a ring 

 cut out of it all round the stem, through the bark and outer 

 layers of the wood, it will be killed, as no water can then pass 

 upwards. As a rule, European species are killed by merely 

 cutting through the bark, as the sugary sap cannot then pass 

 downwards to nourish the parts below the ring. 



The carbon dioxide gas taken in by the leaves is made up 



