620 



After numerous vain attempts, the above described disturbances in 

 tissue, and processes of healing, could at last, in the spring of 1905, be 

 produced artificially. In April potted specimens of 4 to 5 year old oaks 

 w^ere brought into a greenhouse for forcing. The tender young shoots 

 were exposed in May for one night to a temperature of 4 degrees C. below 

 zero in a freezing cylinder. The plants were then left out of doors and 

 investigated the middle of June. Here, exactly as in the observations made 

 the previous year on naturally frozen oaks, the branches, injured by frost, 

 showed very different forms of disturbance. Among them were some 

 resembling typically the natural injuries described above; only the processes 

 of healing, which here begin clearly at the medullary rays, were much less 

 extensive, which may be traced to the fact that potted specimens always 

 develop more weakly and slowly than forest trees growing in open ground. 

 The observation was also made, that the clefts in the tissue seemed to be 

 less extensive, the older and stronger the branch was at the time of the 

 frost action. I conclude from this that injury from frost only leads to 

 the formation of parenchyma wood within an annual ring when it affects 

 very young, tender shoots at the time of the greatest growth in length. 

 Besides this, favorable, warm weather must follow the frosty nights so that 

 cell increase can continue at its former rate. The building material, in the 

 form of mobilized reserve substances, is present in the branch, injured by 

 frost, in the same amounts as before the action of the frost, but the newly 

 produced cell elements develop differently because the conditions of tension 

 in the branch and the resulting pressure on the cambium have become dif- 

 ferent, due to the breaking up caused by frost. 



The Theory of the Mechanical Action of Frost. 



The phenomena, which came to light in the above described natural 

 and artificial frost injuries to young branches, however they may vary, can 

 be traced to simple mechanical processes. In this we still refer to the above 

 illustration of the oak branch in which we see that the pentagonal wood 

 ring, surrounding the medullary disc, passes over suddenly into a light zone 

 of delicate tissue (Ig) and this gradually forms, toward the perihpery, 

 tougher elements, which have the character of normal wood (h). 



The illustrations 2 to 6 in Fig. 149 serve to orient the place of origin of 

 the thin-walled tissue. These show enlarged portions, drawn cell for cell 

 from the right side of the above figure (Fig. 148) at the region of the sec- 

 tion, lying between Ig and b. In all the drawings, the upper angle is the one 

 toward the pith, the under angle the one toward the bark which, in fact 

 (Fig. 149 2, 4, 6) even shows bark elements. The uppermost cell groups, 

 in part designated by h, form the boundary of the wood ring which was 

 present before the action of the frost. These pass over directly into the 

 thin-walled tissue (Ig) of the thin-walled stripe (Fig. 149 2, 5). In this, 

 the medullary rays, which in normal wood are only one to two cells broad 

 (Fig. 149, 5 m s) have become enlarged and irregularly many-celled. They 



