46 Public Parks. 



In like manner, trees act as conductors of heat from the earth in 

 winter, because the surrounding atmosphere is cooler than the eai-th 

 in which they grow. It is true that the conducting power of wood 

 is slow, which is much less transversely, to the direction of its fibre, 

 than with it,* which would prevent the interior of a large trunk from 

 being rapidly affected by the change in the heat of the external air ; 

 and accordingly, it is found that the larger the trunk in which the 

 observation is made, the greater the difference.! Trees possess a 

 specific temperature of their own, independent of their conducting 

 power, — an organic activity for generating heat, like that with which 

 the warm-blooded animals are gifted, though by a different process, 

 which has undoubtedly some influence in estimating the action 

 of the forest upon atmospheric temperature. The range of trees, 

 apart from moisture, is restricted by temperature, and they have the 

 power of withstanding the ordinary changes which take place during 

 the year ; but there are cycles of cold when, in certain species, the 

 internal heat is overcome, and, as a necessary consequence, the life 

 of the tree is destroyed. This is also shown in the protection that is 

 necessary to young trees that are cultivated, to prevent their being 

 destroyed by the ordinary winter temperature, owing to their vitality 

 being insufficient to resist the depressing effect of cold ; and such is 

 also the case with young forest trees, as they will not grow unless 

 protected by other and larger trees. It will therefore be seen that 

 the same law obtains in the vegetable as in the animal kingdom. 



After the leaves fall in the autumn, the rootlets continue to collect 

 sap, and there is no doubt that some, motion of the sap takes place 

 even in winter, although the tree may be said to be asleep, as there 

 is in reality only a diminution in the activity of its vital processes, 

 which is a characteristic of all living beings, some heat must be 

 evolved, which is greatly increased when the sap begins to flow 

 actively and the terminal buds begin to swell. \ It will, therefore, be 

 seen that trees are the source of heat, in addition to the fact that they 

 act mechanically in checking the force and movement of the winds, 

 and thus cause the atmosphere surrounding them to be milder and 

 less subject to sudden changes of temperature. It is the uninter- 

 rupted sweep of the winds, rather than the intensity of the cold, 

 which abstracts from the vital energy of the system. The trapper 



• Dr. Tyndall, Phil. Transact., 1853. 

 t \ Carpenter's Comparative Physiology. 



