38 PHENOMENA OF PLANT-LIFE. 



a portion sometimes remaining attached to the wood sufficient 

 to conduct the elaborated sap and so form a new layer of 

 wood with a layer of bark. The roots are developed from the 

 uninjured portion under the protection of the old bark, and in 

 their nature are precisely like roots from cuttings. An in- 

 teresting specimen from the grounds of Mr.. Charles S. Smith, 

 of Amherst, is exhibited in figure 35. 



The rupture of the medullary rays and separation of the 

 bark from the wood by the combined action of frost and sun- 

 shine is not uncommon in the apple and other cultivated 

 trees. If a severe frost separates the water from the wood 

 as ice, and it then thaws and freezes again before it can be re- 

 absorbed, it will be likely to burst the bark or tissues in 

 which it is accumulated. This usually results in one or 

 more cracks through the bark on the southerly side of the 

 tree, from which there is, in the case of the apple tree, com- 

 monly a slight flow of crude sap in the following April or 

 May. The outside of the bark is blackened, and the detached 

 portions die. 



In the spring of 1874, a vertical crack three feet long was 

 noticed in the 1 south side of a vigorous young Graven stein 

 apple tree in Amherst, the trunk of which was about three 

 inches in diameter. Upon examination, it was found that the 

 bark had not been separated from the thick layer of wood 

 formed the previous year, but that this outside layer was 

 entirely detached from the wood beneath. The bark, being 

 supplied with sap ascending through this layer, remained 

 sound, and, the crack having been filled with wax, the tree 

 grew equally well with others in its vicinity which had sus- 

 tained no injury. The new growth on the sides of the crack 

 being covered only with a thin, soft periderm, will, doubtless, 

 readily unite, and there will soon remain no trace of the 

 rupture. The separated layers of wood, however, will never 

 be reunited, though the inner ones may conduct sap, until 

 converted into the nearly impervious heartwood which oc- 

 cupies the central portion of every trunk after it attains to 

 any considerable size. 



At what age, if ever, the inner wood of exogens loses all 

 power of conveying sap, and whether the sound heart of an 

 old tree which has never been exposed to the influences of the 



