11 



The leaves themselves take in from the atmosphere carbon dioxide gas, and 

 this gas dissolves in the water contained in the leaves . In the leaves, as well as in 

 some other parts of the tree, there is a green-colonred matter called chlorophyll, 

 which has the remarkable property of using the energy contained in sunlight to 

 cause the elements provided by water and carbon dioxide to become united into 

 complex carbon compounds, which form the greater portion of the actual food of 

 the tree. Chlorophyll is itself only found in the presence of light, and an essential 

 element in its composition is iron. The composition of the raw material used in 

 the chemical laboratory of the plant is complex, but, in order that the plant may 

 continue to live, the following elements must all be absorbed in the form of some 

 soluble compound. The essential elements are: — 



Hydrogen. 



Oxygen. 



Carbon. 



Nitrogen. 



Potassium. 



Calcium. 



Phosphorus. 



Iron. 

 Carbon and oxygen make up carbonic acid gas which is absorbed through the 

 leaves, but all other elements must enter as soluble compounds through the minute 

 hairs on the tiny rootlets. Even the largest tree must depend for its food on these 

 fine hairs on its smallest rootlets. The large roots only seem to hold the tree up- 

 right, and to convey .the food materials in solution from the root hairs to the por- 

 tion of the tree above the ground. 



The inorganic chemical compounds which are absorbed by the plant are trans- 

 formed in the leaves into organic compounds, chief among which are carbo- 

 hydrates. This process is known as "photosynthesis," a word which means build- 

 ing up through the action of light. The first stable carbohydrate formed is usually 

 some form of sugar. Photosynthesis only goes on in the presence of light, and 

 often in strong light the production of sugar proceeds at a faster rate than the 

 product can be removed from the leaf to other parts of the tree. The stoppage 

 of the life processes in this way is prevented by the fact that, when the concentra- 

 tion of sugar reaches a certain point, new activities of the protoplasm of the 

 living tree-cells are awakened and the formation of insoluble starch at 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 

 a critical point. Sugars as they arise are also continuously passing away from 

 the leaf to other parts of the tree. They have 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. When the photo- 

 synthetic activity of the leaf ceases at night time, the starch formerly produced in 

 the leaves is re-converted by special ferments into sugar, which thus continues to 

 replace that which is being still withdrawn from the leaves till all the starch may 

 be finally removed 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 they contain 

 little or none. In other words, the process of absorption of food by trees goes 

 on continuously day and night. 



Let us now turn to the mechanics of tree-food. The water from the roots, as 

 has been said, passes upwards into the leaves, in the outer layer of the wood. The 

 sugary sap already referred to travels from the leaves to the parts where it is 

 required in the inner layers of the bark. Hence, if a tree be "ringfbarked" all 

 round the stem through the bark and the outer layers of the wood, it will be killed, 

 as no water can then pass upwards, and it follows, of course, that no life-giving 



