284 GENERAL PLANT PATHOLOGY 



found that the ice fringes are formed when the temperature falls to 

 freezing. They are formed on the outer surface of the plant. The 

 growth of the ice fringe ceases when the ground is frozen to a depth of 

 2 to 3 cm. and when the moisture in the stem is frozen. The dimen- 

 sions of the fringe depend upon the rate of evaporation of water from 

 the stem up which it rises by capillary action and upon the amount 

 of moisture in the ground. Clouds and fogs in some regions have an 

 important effect on vegetation.^ The two forms of foliage leaves on 

 the branches of the redwoods of California are conditioned upon the 

 height of the fogs which drift in from the Pacific Ocean. The leaves 

 on the fog-exposed branches are flat and divergent, while those on the 

 sun-exposed branches above the fog level are scale-like and appressed. 

 The London fogs work detrimentally to outdoor and greenhouse plants, 

 and in Egypt, the cotton capsules long exposed to fog are more in- 

 fested with black moulds. Dew, which lodges on the margins of leaves, 

 is responsible for the entrance of fungi by their spores lodging in the 

 dewdrops and germinating there. 



The weight of snow and ice breaks off the limbs of trees, breaks down 

 herbaceous plants, and this opens up the way for the entrance of 

 various parasitic fungi. Ice or sleet storms are especially severe at times 

 to trees. The year 1902 was noted for two exceptionally destructive 

 ice storms which visited the Philadelphia region. One of these storms 

 occurred on Friday, Feb. 21, and the other on Saturday, Dec. 13.^ The 

 storm of Feb. 21 was accompanied by high winds and did an irreparable 

 damage to the fruit, forest and shade trees. Meteorologically speaking, 

 regions of strongly variable temperature are subject to occasional 

 winter storms in which the precipitation occurring as rain, freezes as 

 soon as it touches any solid body, such as the branches of trees, telegraph 

 wires or the ground. This happens when the ground and the lower 

 air have been made excessively cold during a spell of clear anticyclonic 

 weather, when a moist upper current in advance of an approaching 

 cyclone brings clouds and rain. All our meteorologists prefer to call 

 such storms ice storms; locally near Philadelphia they are denominated 

 sleet storms. The weight of ice which such limbs carry is astounding. 



1 Weiss, F. E., Imms, A. D., Robinson, W.: Plants in Health and Disease, 

 1916; 54-56. 



" Harshberger, John W. : Relation of Ice Storms to Trees. Contrib. Bot. 

 Lab. Univ. of Penna., II: 345-349, 1904. 



