577 



part of the trunk shows a regular, brown figure. Most well-known is the 

 "Landzvchr cross" in the maple and similar structures in Cytisus and 

 Fraxinus. Cytisus and other Papilionaceae show at times a very attrac- 

 tive bright coloration of the cross-sections which should be made of 

 practical use. The bright coloration is caused by the different degrees of 

 browning in the heart wood and cambial zones. 



Such regular, surface-like discolorations are of rare occurrence. The 

 most frecjuent phenomenon consists of an irregular browning of those parts 

 of the bark which surround a bud and of those outcurvings of the pith 

 which lead to this bud. The amount of tissue disease naturally depends 

 upon the time and intensity of the action of the cold as w^ell as the specific 

 sensitiveness of the variety and, with equal intensity, on the age of the axis. 

 As a rule, the younger a branch is, the more extensive is the tissue browning. 



The cross-section, shown in Fig. 128, of a pear branch injured by 

 artificial frost, gi\es an insight into the variety of browning due to frost. 

 In this, m indicates the pith body, m k, the pith crown, ;;; h, the outcurving 

 of the pith disc, called pith bridges, which lead to the bud, lying close above 

 this section but not visible in it. At the place where the bud lies, each 

 stem is more or less thickened and pushes out from a "bud cushion." In 

 this bud cushion lie also the vascular bundles //' and (/", which pass down- 

 ward into the petiole, in the axil of which is found the bud. The cap of 

 tissue, which, in the drawing lies above the central cord of the leaf spur, 

 and seems laid against the bark body of the twig, represents the cicatriza- 

 tion tissue which had formed in the previous year after the falling of the 

 leaf. The different ducts in the cords of the leaf spurs and in the wood 

 ring are distinguished by g,' g" and //. The wood ring (h) wdth the medul- 

 lary rays (ni s) shows diverse, predominantly radial clefts, while the tissue 

 openings (/) in the bark tissue usually extend tangentially. Noteworthy is 

 also a gaping, longitudinal split which breaks the pith bridge and, by the 

 amount of injury, makes apparent that it represents the part of the branch 

 most susceptible to frost. 



In many deciduous trees there is still a second region of great sus- 

 ceptibility to frost, viz., the hard bast cells and their outer parenchymatous 

 covering. In my experiments w^ith artificial freezing, cherries, plum^, red 

 beeches and apples were proved especially susceptible, while pears showed 

 a greater power of resistance. In the adjoining picture the bast bundles 

 (b) are found to be unattacked. just as little is the collenchyma (cl). 

 The cambium zone (c) which, by its brown color, indicates to tree breeders 

 in the spring pruning of fruit trees that the branches have been injured by 

 frost, has not been browned in the pear. In microscopic investigations, it 

 is found that usually the still cambial, thin-walled, young wood and the 

 innermost young bark, of the same age, have been browned, while the 

 meristem layer, rich in cytoplasm and lying between both regions, appears 

 colorless and uninjured. 



