532 RELATION TO ENVIRONMENT. 



different heights, the canopy is said to be " compound" or 

 "storied." Where it is uneven, there are open places in the 

 canopy which admit more light, in which case the undergrowth 

 may be different. The interior of the forest lies between the 

 canopy and the floor. It provides for aeration of the floor and 

 interior occupants, and also room for the boles or tree trunks 

 (called by foresters the wood mass of the forest) which support 

 the canopy and provide the channels for communication and 

 food exchange between the floor and canopy. The canopy 

 manufactures the carbohydrate food and assimilates the min- 

 eral and proteid substances absorbed by the roots in the soil; 

 and also gets rid of the surplus water needed for conveying food 

 materials from the floor to the place where they are elaborated 

 It is the seat where energy is created for work: and also the 

 place for seed production. 



1011. Longevity of the forest. The forest is capable of self- 

 perpetuation, and except in case of unusual disaster or the action 

 of man, it should live indefinitely. As the old trees die they 

 are gradually replaced by younger ones. So while trees may 

 come and trees may go, the forest goes on forever. 



1012. Longevity of the tree. Trees, like nearly all organ- 

 isms, pass through the stages of growth and senescence, and 

 then die. What it is which inhibits the growth of trees at a cer- 

 tain stage, and later brings about death, is not known. The 

 entire tree is surrounded by a layer of growing tissue (cambium). 

 The branches and roots are extended in length by increase of 

 this tissue at the " growing points," while the trunk, branches, 

 and roots are increased in diameter by the growth of the cylinder 

 of cambium underneath the bark, which adds each year a layer 

 outwardly to the bark and a layer inwardly to the wood. Thus, 

 each year, new embryonic tissue; new channels for conduction 

 of food; new organs for absorption, for respiration, and for 

 photosynthesis, assimilation, transpiration, reproduction, etc., 

 are formed. For this reason some have suggested there is no 

 reason why the tree should not live forever, barring accidents. 

 But, when trees reach a certain height, increase in height ceases, 



