644 



the tissue. Jonsson's^ experiments have proved that the bubbles in the 

 glass are actually the cause. He observed the light image of the sun's rays 

 produced on the leaf by such bubbles and the changed position of such 

 spots resulting from the change of the sun's position. This explains also 

 the not infrequently observable phenomenon that such burned spots appear 

 in regular lines. 



One experiment proved, however, that sprinkling can also act danger- 

 ously, when a drop of water remained hanging on the under side of the 

 cover glass, fastened at some distance above the surface of the leaf. In 

 this, traces of burned spots could be produced, while drops of water lying 

 directly on the leaf caused no injury. 



To avoid such disadvantages, it would be advisable in general practice 

 to choose better grades of glass at least for those hot houses in which valu- 

 able foliage plants are kept. 



Defoliation. 



Phenomena of scorching are not here concerned but rather the precipi- 

 tous maturity of the tissues. In cases observable out of doors, a great 

 dryness of the soil is usually combined with the direct action of the sun. 

 Special experiments with burning glasses show, however, that even in 

 damp soil the leaves are thrown oflf which are most strongly injured by 

 burned spots. Wiesner- found that, in "the falling of leaves due to heat," 

 those which usually fall come from the inner part of the crown of the tree, 

 rather than from its periphe^\^ He thinks that these outer leaves, as a 

 result of their greater radiation of heat, do not become so warm as the 

 leaves found in the enclosed places. We might seek the reason for this in 

 the different vitality of the organs. Those exposed to the greater amount 

 of light produce more substance and their cells are richer in cytoplasmic 

 material. They have, therefore, with an abnormally increased evaporation 

 and respiration, more reser\^e substances and are longer lived than leaves 

 of the same period found in the inner part of the tree crown. Young 

 organs in themselves are more resistant. 



In cases occurring out of doors, the place of growth, together with the 

 water supply, acts decisively. Among forest trees, this is seen best in oaks 

 and larches in young plantations where individual specimens, already show- 

 ing completely dried bunches of leaves, are always to be found betv/een 

 green trees which have been uninjured, or only slightly changed. 



In one young larch plantation, I found that the specimens most greatly 

 injured had lost almost all their needles from the upper branches. Onlv the 

 very young shoots, the tips of which seemed twisted and a fox red, still held 

 needles which hung downward like red tassels. The youngest needles of all 



1 Jonsson, Benst, Om Brannflakar pa vilxtblad. Botaniska Notiser 1891. 

 Zeitschr, f. Pflanzpnkrankh. 1892, p. 358. 



2 Wiesner, Jul., t)ber den Hitzelaubfall. Ber. d. D. Bot.. Ges. 1904, Vol. XXII, 

 p. 501. 



