282 GENERAL PLANT PATHOLOGY 



their leaves burned if part of the whitewash is removed. The light 

 passes through the opening thus made and the leaves on which it is 

 concentrated are scorched. 



Several diseases of plants are caused by too brilliant sunlight. Such 

 are sunscald, sunscorch and bronzing.^ Sunscald may follow as a 

 result of too intensive sunlight, as, for example, when certain fruit trees 

 are stripped of their foliage in summer, such as sometimes results from 

 the ravages of the gypsy moth. In such instances the new unripened 

 wood sunscalds badly. Sometimes it is associated with severe and 

 abrupt changes in temperature on non-ripened wood. "Sunscorch" 

 is a term applied to the burning of foliage in summer during periods 

 when the soil is dry, and is also common to evergreens during warm 

 wiildy days in spring before the frost is out of the ground. Any 

 defects in the root system which prevent root absorption may cause 

 sunscorch. "Bronzing" of leaves is a form of sun scorch characterized 

 by the occurrence of a reddish-brown or bronze color of the leaf. It is 

 caused by a lack of soil moisture, or defective root absorption during 

 dry, hot periods. 



Too much shade is also detrimental to plants, as is seen under the 

 dense canopy of beech trees on a lawn, where nothing will grow, not 

 even a blade of grass. The grasses, etc., die of inanition. The condi- 

 tion known as etiolation originates where a plant is grown in the dark, 

 or in subdued sunlight. Growth in darkness leads to important modi- 

 fications in the general habit and structure of a plant. If we take a 

 potato plant and raise it in the dark, we find the etiolated shoot has 

 a white stem and leaves which are at first pinkish, and subsequently 

 pale yellow, and the absence of chlorophyll is noteworthy. The inter- 

 nodes are long and slender and the leaves are small compared with the 

 green plant and there are corresponding anatomic differences. Morn- 

 ing glories raised in greenhouses in the winter do not twine. They 

 grow from four to five inches tall and have only one to two flowers. 



Heat as a factor in the growth of plants is well known. Each plant 

 has its minimum, maximum and optimum degree of heat. The dis- 

 tribution of plants over the larger stretches of the earth's surface is 

 associated with the amount of heat that the different plants receive. 

 The absence of heat, where the plant is exposed to a temperature below 



1 Stone, George E.: Injury to Vegetation Resulting from Climatic Conditions. 

 Monthly Weather Review, 44: 569-570, October, 1916. 



