298 Report of the Botanical Department of the 



He says that sun-stroke is the most destructive disease of pear 

 trees. " The tender, smooth bark of a growing pear tree, being not 

 yet hardened, when struck directly by the solar rays, separates 

 longitudinally from the alburnum to the extent of from two to five 

 inches. The bark cracks in the middle, and its edges curling up, it 

 affords a refuge for insects, which take up their quarters there and 

 contribute by their biting to increase the size of the wound, and to 

 produce a canker which most frequently causes the destruction of 

 the tree." He advocates the covering of the tree stems to protect 

 them from the sun's rays, as is also done by some of our modern 

 horticulturists. 



Caspary^ quotes de Jonghe's last article in full and calls attention 

 to some cases of low-temperature injuries which result in the sepa- 

 ration of the bark from the wood, but attributes them to sudden 

 freezing of the sap in the cambial region in spring. 



In a further contribution to the cause of wood clefts in trees Cas- 

 pary^ confirmed his earher findings in part by experiment. He 

 found that the width of the clefts is proportional to the degree of 

 cold; and that clefts in smaller trees will open quicker when the tem- 

 perature becomes low, and close sooner when the temperature rises 

 than those in larger ones. He also noted that old clefts reopen when 

 the temperature is only a few degrees below freezing while new clefts 

 do not form until a considerably lower temperature is reached. 



The circumferences of short disk-like sections of tree trunks were 

 carefully measured and then after some of them had been cut radially 

 from the bark to the center with a saw, they were exposed to different 

 temperatures and remeasured. The radial saw cuts which had closed 

 on removing the saw were found open when the temperature had 

 sunk a little below freezing, and two uncut sections were cleft over 

 night by a temperature of — 7.2° C. 



Measurements showed that the contraction and expansion of 

 these sections as well as the opening and closing of the radial clefts 

 followed the temperature changes just as the dimensions of the cir- 

 cumferences and clefts did in trees: when the temperature rose their 

 circumferences increased and the clefts closed, while lower tempera- 

 ture decreased the circumferences and opened the clefts. He also 

 found that the circumference of the bark changed more rapidly than 

 that of the wood cylinder. 



From these results Caspary concludes that the coefficient of expan- 

 sion of tree trunks is even greater than that of ice, zinc or iron; and, 

 that trees are cleft because their circumferences decrease more rapidly 

 than their radii when the temperature is lowered. 



' R. Caspary. Bewirkt die Sonne Risse in Rinde und Holz der Baume? 



Bot.Zeil. 15:153-56. 1857. 

 2 R. Caspary. Neue Untersuchungen iiber Frostspalten. 



Bot. Zeit. 15:329-35; 345-50; 361-71. 1857. 



