4 FORESTRY IN NEW ENGLAND 



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 dioxide of the atmosphere through 

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

 carbon in 10,000 Hters of air, one tree with a dry weight of 5000 

 kilograms requires about 1 2 million cubic yards of air to furnish 

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

 resupplied with carbon exhaled by animals and poured forth 

 from factories and chimneys these figures would seem startling. 

 The process of assimilation, or the taking of carbon from the 

 carbon dioxide, 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 night and the starch grains are dissolved and pass out 

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

 are stored. This surplus is greatest at the end of the growing 

 season, and upon its amount depends all growth of the next year 

 either of mother-plant or offspring in germinating seed. This 

 material is stored in the form of starch or sugars in the embryo 

 of the seed, in tubers, bulbs, roots, and medullary rays of the 

 wood. 



All trees require a certain amount of light in order to carry 

 out this work of assimilation. With regard to their ability to 

 bear shade, marked differences occur among the various species 

 of trees, so that they may be classed as: light-demanding or 

 intolerant trees, such as larch, red cedar, and gray birch; and 

 shade-enduring or tolerant species, such as beech, spruce, hem- 

 lock. Between these is a class of intermediate trees, including 

 white pine and chestnut. The demands of various species for 



