566 



extensive tree plantations, a deejjcr jjcnelration of tlic cold can he pre- 

 vented by covering the loosened soil. It is a frecjuent but not universal 

 discovery that the roots of recently transplanted trees suffer more from 

 winter frosts than do specimens left in their orig^inal jtlace of ,<^ro\vth. 



Frost Clefts. 



The temperature inside strong tree trunks can ff)llo\v the temperature 

 of the outer air only slowly and thus the inner part of the trunk is colder 

 than the surrounding air from morning until noon but is warmer in the 

 evening\ Then a contraction of the tissues, due to the appearance of cold, 

 will manifest itself in the outer layers of the trunk while the core still 

 retains its original distension. In this way, dit'ferences in tension arise 

 which become the greater, the shari)er the change in temperature. Xow , 

 with a fall in temperature, the wood body contracts more stronglx- in the 

 direction of the circumference, i. e., tangentially, rather than radially, so 

 that the peripheral mantel of the still warmer core of the trunk really 

 becomes too tight. It must accordingly be stretched tangentially if it shall 

 still entirely enclose the core. If, with increasing cold, it can not stretch 

 sufficiently, it must split. In this way tears are produced in the bark of 

 the tree which advance the deeper into the wood, the greater the cold and 

 the difference between the cooled peripheral and the warmer central tissues 

 of the trunk. In great, sudden cold, it has been found that some tree 

 trunks crack audibly and then show hjng, deep gaping splits following the 

 twisting of the wood fibres. Some varieties of trees show this phenomenon 

 especially frequently. The horse chestnut suffers most of all; besides this, 

 the oak, poplar and cherry should be especially emphasized. The cleft 

 remains gaping only so long as the heavy cold lasts. With the apjjearance 

 of warmer weather the edges of the split approach one another, even closing 

 the wound entirely. The wound, however, is almost ne\er well healed and 

 breaks open again the following winter. The process of healing is normal, 

 since circumvallation rolls are formed from the cambium, the young wood 

 and the bark, and strive to unite. In other injuries with free lying wound 

 surfaces, these projecting overgrowth edges, howcxer, do not find the space 

 necessary for their extension, but are forced to grow directly against one 

 another and to i)ush out over the edge of the cleft wound. They therefore, 

 from mutual pressure, form rolls projecting outward, depressed in the 

 centre like lips, which are called "frost ridges.' 



In Fig. \22 we see such a frost ridge on a strong trunk of Acer cam- 

 pcstrc, which shows a number of radial clefts. One of these has split 

 through the stem, so that an outwardly \isil)le cleft has been [)roduced 

 which, at the beginning, gaped widely but, with the ai)pearance of warmer 

 weather, has become very narrow. WHien in spring the tree would have 

 closed the split by growth of the cambial layer, the circum\allation edges 



1 Squires, Roy \V., Minnesota Dot. Studies. Bull. 9, 189J 



