575 



in the intercellular spaces, contracts the tissue, and so much the more the 

 thinner walled the tissue is. The bark suffers considerably more than does the 

 wood which, reached later, cools down less easily and contracts less. The 

 tangential contraction is greater than the radial. This difference acts likes 

 a one-sided strain, taking place in the direction of the circumference of the 

 trunk to which the dift'erent layers of the bark will respond to a different 

 degree, when the bark, as a \\hole, is Aery young. \\'ith an equal amount 

 of contraction at all points in the bark, the cells lying nearest the periphery 

 and most elongated in the direction of the circumference of the trunk, will 

 be most displaced. If one considers that the outer cells of the primary 

 bark, because of the great coarseness of their walls, are not as elastic as 

 the underlying, thinner-walled ones, it is clear that, when the strain ceases 

 in them the permanent increase, caused by the incomplete elasticity, will be 

 ihe greatest. 



After the frost action has stopped (it continues only a short time in 

 late frosts ) the increased turgor will cause the cells to retain their distended 

 form and, since the outer, greatly distended bark layers no longer have 

 sufficient space in the previous tangential plane, they become raised in 

 wrinkles or blisters above the plane of the circumference of the trunk and 

 in this way form "frost wrinkles." 



Besides the tangential and radial contraction, there is an additional, 

 longitudinal change in the young, still herbaceous twigs, which must be 

 produced with the twisting of the axillary body, caused by the frost action. 

 One can easily produce cross wrinkles artificially on one year old shoots 

 by bending them. Reference should be made to a work by Ursprung' in 

 regard to the tensile conditions developing in bent, herbaceous stems. 



Bark Tatters and Cork Holes. 



The loosening processes which set in after a drying of the outermost 

 tissue layers when the branches have been killed by frost occur more fre- 

 c|uently than the phenomena of raised bark, appearing in the form of frost 

 wrinkles and blisters in living bark tissue. In Fig. 127 is shown a branch 

 with loosely rolled back, flapping, dry bark tatters on the autumn (Sylves- 

 ter) pear. Even in the soft wooded apples (Morning Breath) the 

 l)henomenon was found in May and June, on branches and young, still 

 smooth barked, sapling trunks. The periderm is seen at first to be dis- 

 tended in blisters ; later the blisters split longitudinally. Th.e whole bark 

 l)arenchyma underneath the tear seems blackened and dries up quickly. 

 The death of the bark tissue advances further, the more the tear widens, 

 since it at first becomes yellowish green and tender, then grows dark, col- 

 lapses and finally dies. 



In time these dead spots become entirely bare, since the longitudinal 

 tear in the periderm l)lister lengthens and new cross tears divide the whole 



1 Ursprung-, A., Beitrag ziir Krklarung de.s exzentrisclien Diekenwachstuin.s an 

 Krautpflunzen. Ber. d. D. Bot. G. 1906, Part 9, p. 498. 



