LIGHTNING. 477 



tion primarily affects the cell-contents, and later the cell-wall. 

 It is very highly probable that in certain cases various pro- 

 ducts of decomposition arising from the progress of the fungi may 

 themselves prove poisonous to contiguous parts of the plant. 



One of the most important problems of practical horticulture 

 and agriculture is the search for efficient means by which invad- 

 ing fungi may be destroyed without at the same time injuring 

 the host-plant to which they have attached themselves. 1 



1245. The presence of certain fungi in plants sometimes gives 

 rise to abnormal growths and to various distortions. When once 

 their disturbing influence is felt, the subsequent growth may be 

 affected for a long time, and the malformations become of an 

 extraordinary character. 



1246. Considerable distortions are often produced by bites 

 or other injuries by insects. 2 Galls for instance those of the 

 oak and willow are among the most noteworthy instances of 

 this kind. 



1247. The effects of lightning upon trees have been examined 

 by many observers. Cohn 8 and Colladon* have pointed out 

 some of the characteristic injuries sustained by species of poplar, 

 elm, and oak, stating that the stroke does not usually affect the 

 summit of the first two, but that oaks are frequently struck at 

 their uppermost branches. The course of the injury is often 

 spiral, winding around the trunk in stripes which involve part of 

 the sap-wood and bark. 



It is not now believed that any species of trees are exempt 

 from injury from lightning, although the ash was formerly 

 thought to " possess a remarkable degree of immunity. 



1248. Partial or complete blanching of otherwise healthy leaves 

 exposed to light has been regarded by some observers as an indi- 

 cation of a diseased condition. In some cases the blanching is 

 dependent upon a lack of iron in the soil (see 791), but in others 

 it appears to be strictly hereditary, being propagable both by 

 bud and by seed. Nothing is known, however, as to its causes 

 in these cases, and they are generally referred to the unsatis- 

 factory category of sports. 



It is worthy of notice that a considerable proportion of the 

 so-called variegated plants, especially of those which have only 



1 For an account of some experiments in this field, see Frank : Pflanzen- 

 krankheiten, 1879 ; and Nobbe : Handbnch der Samenkunde 



2 For a bibliography of this subject, see Frank's Pflanzenkrankheiten. 

 Denkschrift. d. Schles. Ges. f. vaterl. Knit. Breslau, 1853, p. 267. 



* Mem. de la Soc. de Phys. et d' Hist. Nat. de Geneve, 1872, p. 501. 



