8 BULLETIN 1131, U. S. DEPARTMENT OF AGRICULTURE. 



along this entire row of Pinus densiflora trees, which was the outer- 

 most row on one side of the nursery, had wilted and drooped, due 

 to loss of turgor. On a row of trees of Taxus baccata near by in 

 the same nursery it was observed that a large number of the new 

 shoots, which averaged half an inch in length, were killed outright. 



On September 30 the writer had the opportunity to observe these 

 trees. The entire row of Pinus densiflora trees showed numerous 

 oases of permanent deformation of the terminal and many of the 

 lateral shoots of the last whorl, but all had remained living and 

 had regained to a large extent their normal erect position, although 

 not without leaving more or less of an S-shaped kink in their stems 

 (PL III). In all cases which the writer examined, such shoots ex- 

 hibited a frost ring in the beginning of the 1921 growth ring, which 

 could be traced readily down the stem for several inches from the 

 base of the last whorl, although it was scarcely to be distinguished, 

 even with a hand lens, from the outer limit of the 1920 growth ring, 

 owing to its close coincidence (PI. IV. A), The frost ring like- 

 wise was traceable macroscopically on sections cut with a keen micro- 

 tome knife, though better microscopically, for a distance of several 

 centimeters above the bases of the deformed terminal and lateral 

 shoots of the last whorl, where it appeared in the first wood elements 

 bordering upon the pith and in the outer cells of the pith. It was 

 lacking in those few lateral branches of the last whorl that some- 

 times escaped injury by reason of their lack of development at the 

 time of the freeze. In the far less numerous cases of frost injury 

 in Taxus baccata. however, there was no evidence of any deforma- 

 tion of the young shoots, such as occurred in Pinus densiflora . but 

 the young terminal shoots were killed outright and replaced by one 

 or more volunteer shoots. In all such cases, where the terminal 

 shoot had been killed by late frost, a frost ring could be traced down 

 the stem for several inches below the base of the dead terminal shoot 

 in the beginning of the 1921 growth ring. 



A row of trees of Pinus rhontoma var. wncinata. a more hardy ap- 

 pearing species planted next to the row of Pinus densifiora, showed 

 no single external symptoms of frost injury, and none of the shoots 

 which the writer cut into showed any frost rings. On the other hand, 

 with the exception of the two species mentioned, there was no evidence 

 of any deformation or killing of the shoots by frost on any of the 

 other conifers, of which a large variety were in the nursery. 



ANATOMICAL STRUCTURE OF THE FROST RINGS. 



As is well known, when living plant tissue is frozen the water is 

 withdrawn from the cells, solidifying to ice in the intercellular spaces 

 or other tissue gaps. Upon the initiation of the freezing the water 

 from the still living cambial wood passes out between the wood and 

 the bark and forms an ice mantle there. The extraordinarily tender 

 nature of the youngest cambial cells favors the separation of the 

 tissue, and a loosening of the phloem is facilitated either by the 

 stronger shrinkage of the frozen wood or by the expansion of the 

 cortex due to the stress exerted by the ice formation beneath it. The 

 lower the temperature falls the thicker the ice mantle^ becomes, and 

 it compresses the tender cambial cells until their outlines are more 

 or less indistinguishable, as is to be seen in Plate IV. A, B. and C. 



