FORMATION OF FROST RINGS IN CONIFERS. 9 



The already thick-walled but still unlignified cells collapse also, 

 their walls presenting a crumpled appearance (PI. II, B ; PL V, B). 



After the thawing, the cell tissue that has been compressed does 

 not expand to its previous form and size, but remains permanently 

 distorted. In the cases of the more severe injury there begins at the 

 periphery of the wood formed before the injury a more or less broad 

 zone of large-celled parenchyma, which is distinguished by its 

 greatly thickened simple-pitted walls and by the dark-brown color 

 of the walls and the cell contents. This zone of parenchyma tissue 

 quickly passes over into tracheidal tissue, which at first is usually 

 somewhat larger celled than that developed before the frost injury, 

 but which quickly becomes typical. In this manner the frost injury 

 results in the formation of a false ring, especially if it occurs after the 

 development of several spring-wood tracheids (PL IV, B ; PL V, 

 A; PI. VI, A and B). 



As may be seen from the accompanying reproductions of photo- 

 micrographs, the frost rings exhibit great dissimilarity in structure, 

 according to the degree of intensity of the frost action and the sus- 

 ceptibility of the wood tissue at the time of its occurrence. 



The medullary rays, which extend through the frost ring and 

 stretch in accordance with the stress exerted upon them, naturally 

 suffer most from the displacement of the tissue. Their deformation 

 varies according to the severity of the injury, but in general is, 

 very characteristic. On the inner side of the frost ring the rays 

 widen out abruptly, often becoming 2-seriate or 3-seriate instead of 

 uniseriate (PL IV, B\ PL II, B). The rays apparently are stimu- 

 lated to lateral broadening by the diminution of the pressure nor- 

 mally exerted hy the adjoining wood elements, caused by the crush- 

 ing together of the young wood elements. This broadening ensues 

 immediately in the region of the frozen young wood and reaches 

 its greatest extent within the region which, in the frozen condition, 

 was filled by ice. In addition to broadening out laterally, the rays 

 usually are also more or less sharply displaced, often undergoing 

 a knee-shaped bending (PL II, A and G). Within any one stem 

 the medullary rays are usually, although by no means always, dis- 

 placed uniformly either to the one side or to the other. As the wood 

 ring enlarges after the thawing, the medullary rays are brought 

 into an oblique position and later grow out again in their original 

 direction, continuing in equal number in the newly formed wood 

 and causing the wood tissue to appear as though a fault had oc- 

 curred in it. The lateral displacement of the medullary rays appar- 

 ently depends upon the circumstance that their stretching during 

 the ice formation remains preserved after the thawing. This lateral 

 expansion and displacement of the medullary rays is by far the 

 most conspicuous and characteristic feature of late- frost injury and 

 is a constant feature of all injuries to wood by late frost. In at 

 least the more severe cases of injury the frost ring is further accentu- 

 ated by a more or less broad zone of brownish parenchyma tissue. 



There also may arise after the thawing a series of radial gaps 

 or clefts, occurring with variable frequency and conspicuousness 

 within the tracheidal tissue, where it had been stretched apart pre- 

 viously by the excessive tangential contraction. With subsequent 

 growth, these tissue gaps become filled with large-celled parenchyma 



