100 ORGANIC STRUCTURE. 



In old trees it is only a few of the inner layers that 

 participate in the vitality, or act organically in the 

 system. All the outer layers serve only as a cover- 

 ing: and these, if not thrown off, or cracked into 

 longitudinal fissures, or stretched horizontally to 

 make room for the internal accretion, actually check 

 the growth and hasten the decay of the tree. 



Bark, in the generality of trees, is thicker or 

 thinner according as the growth of the stem has 

 been rapid or slow. Trees that stand singly, or on 

 elevated situations, and consequently exposed to full 

 air and every wind that blows, increase in diameter of 

 stem and extent of branches much more in propor- 

 tion than if they had been drawn up in close planta- 

 tions or in sheltered places. Hence each year's layer 

 of liber partakes of the character of the stem, not by 

 an increase of number of layers, but by an augmented 

 thickness of each. 



The leaves and other appendages of the bark need 

 not be described here ; suffice it to notice only, that 

 their expansion is cotemporary with the development 

 of the other parts of the stem mentioned above. 



Pendulous stems. These are commonly called 

 weeping trees ; and are exemplified in the Betula 

 pendula, and the Salix Batylbnica. The young 

 shoots, instead of being erect, obliquely or horizon- 

 tally spreading, droop and hang almost perpendicu- 

 larly. This is evidently a consequence of rapid 

 growth and constitutional laxity. The erect position 

 of the shoots of other trees depends on their mode- 

 rate growth and rigidity. Those of weeping trees 



