4 A MANUAL OF FORESTRY 



on water than the broad-leaved genera. This is the reason why 

 sand plains, such as those of Gape Cod, can support the pine. 

 The water table is always found to be farther removed from the 

 surface of the ground under forest than outside it or under a cut- 

 over forest; under old forests it is lower than under young 

 -lands. Seedlings spring up in a forest when the roots of the 

 adjoining trees are cut through, thus allowing the water table to 

 come nearer the surface. From the illustration of the oak 

 mentioned above it will be readily realized, in dry seasons 

 especially, that the amount of water available per tree might 

 well be the controlling factor influencing growth or life itself. 

 Forests transpire more moisture than other vegetation under 

 the same conditions. Any deficit caused by excess of transpira- 

 tion over precipitation is necessarily made up by water from the 

 open. Other things being equal and leaving the surface out 

 of account, the soil under a mature forest is drier than in the 

 open. Thus seedlings grown in the open get more moisture in 

 a dry season than under large trees, but, on the other hand, the 

 intense heat in the open may more than counterbalance the 

 greater supply of water. 



The chief element of dry wood is carbon, which forms about 

 one-half the dry weight of plants. This element is entirely 

 absorbed from the carbon dioxid of the atmosphere through 

 the process called assimilation. As there are only two grams of 

 carbon in 10.000 liters of air, one tree with a dry weight of 5000 

 kilograms requires about 12 million cubic yards of air to furnish 

 the carbon. But for the fact that the air is being constantly 



plied with carbon exhaled by animals and poured forth 

 trom factories and chimneys these figures would seem startling. 



process of assimilation, or the taking of carbon from the 

 carbon dioxid. can only be carried on by the chlorophyll or green 

 bodies in the leaves, by the action of sunlight, and a definite 

 amount of heat. The first products of assimilation are carbo- 

 hydrates, either in solution or as starch grains. The process 

 ceases at ni^ht and the starch grains are dissolved and pass out 

 of the cell. The surplus products that are not needed at once 



