28o GENERAL PLANT PATHOLOGY 



the plant. As an illustration of this may be cited the planting of the 

 Carolina poplar on the open porous sandy soils of New Jersey. About 

 Philadelphia, where the tree is largely planted, it grows rapidly with 

 a dense crown of dark-green, foUage leaves. In New Jersey, it grows 

 less rapidly, its crown is more open by a wider spacing of the branches 

 and the leaves have a greenish-yellow appearance and drop off earlier 

 in the autumn than similar trees on the Pennsylvania side of the Dela- 

 ware River. This difference is without doubt associated with the water 

 requirements of the tree, for on the Pennsylvania soils, it can secure 

 abundance of water during the growing season, while in the New Jersey 

 sands, owing to their porosity and the rapid drainage of water through 

 them, the Carolina poplar does not receive sufficient amounts of water 

 for its most vigorous growth. 



The experiments of Miinch^ throw important Ught on the content 

 of water and air in the tissues as a determining factor of disease of 

 woody plants, such as on forest and fruit trees. He has shown that the 

 greater number of the wood-destro^ang fungi require a large amount of 

 air and are able to grow only when a maximum amount is present. The 

 air content of the tissues is dependent on the water supply and trees 

 with narrow annual rings are more resistant than those with broad ones, 

 because the former contain more water and less air relatively. Differ- 

 ent annual rings of the same tree may be attacked differently. 



The decayed rings of wood in such trees are alwajrs the broad ones. 

 The tissues of vigorous branches are rich in water and poor in air and 

 infections do not alwa3rs penetrate to such regions. The healthy bark 

 of beech trees in winter-rest contains 19 to 20 per cent, of air and at 

 the time of budding the air diminishes to 11 per cent., rising afterwards. 

 This is correlated with the canker disease, Nectria ditissima, which in 

 Europe does its damage during the winter months, while during the 

 vegetative period it ceases. Hence, we have opened here a very profita- 

 ble line of investigation to determine the relative amounts of air and 

 water with respect to immunity, or its absence. Again, in the irrigated 

 districts of America, the fruit trees have only a few diseases due to 

 species of Valsa and other species of fungi. Defective irrigation may 

 bring about the prevalence of the die-back diseases, which may be reme- 



» Munch, E.: Untersuchungen uber Immunitat und Krankheitsempfanglich- 

 keit der Holzpflanzen. Naturwiss. Zeitsch. f. Forst. und Landw., 7: S4-7S, 87- 

 114, 129-160, igog; AppEL, O.: Phytopathology & Scientific Botany, loc. cit.' 



