GENERAL CONSIDERATION OF PLANT DISEASES 251 



died by changing the system of irrigation. The land should be irri- 

 gated at the time when the trees contain small amounts of water and 

 much air, so as to prevent an excessive decrease of water in the tissues. 



The condition of the humus has a rather remarkable influence on 

 the growth of plants. Ericaceous plants, such as the trailing arbutus 

 {EpigcEa repens), wintergreen {Gaultheria procumbens), bearberry 

 (Arctostaphylos iiva-iirsi), blueberry {Vaccinium corymhosum) flourish 

 in an acid humus and if the attempt is made to grow such plants 

 under other conditions, they languish and die. Other species like In- 

 dian turnip {Ariscema triphylla), blood root (Sanguinaria canadensis), 

 rue anemone {Anemonella thalidroides) grow best in a leaf- mould humus 

 which is neutral or slightly alkaline. Reverse the reaction of the soils 

 about these plants and they gradually die. 



The presence of an impervious hard pan below the surface soil is 

 a condition which prevents the normal development of trees, as I have 

 shown in my book on the "Pine Barren Vegetation of New Jersey," 

 where in the region known as the Plains, the pitch-pine trees are kept 

 dwarf owing to an impervious subsoil layer. There the trees flourish 

 for a number of years, then begin to suffer until unable to penetrate the 

 deeper layers of the soil, they finally succumb to be replaced by younger 

 trees which meet the same fate. 



Climatic and Meteorologic Factors. — The most important 

 climatic factors, which may be looked upon as in any way related to 

 disease production, are light, heat, precipitation (rain, dew, frost, snow, 

 hail and ice) wind and electricity (lightning, etc.). 



Light is essential for the life functions of all green plants. Carbon 

 dioxide and water are united by the energy of sunlight to form starch. 

 The synthesis takes place in the chloroplast, starch being formed as 

 the first visible product. Ordinary sunlight of a bright, clear day may 

 under certain conditions of plant growth be too intense and it acts 

 prejudicially. The writer has frequently noted, that garden plants 

 suffer, if after a wet, cloudy spell during the rapid period of growth, 

 they are exposed to a bright sun without protection. It takes a few 

 days of bright light to sun-harden the plants. Trees, especially with 

 a smooth bark, which have grown in a very dense wood, and then 

 suddenly isolated in later life, suffer from scorching of the cortex. 

 They are sunburned. Plants grown in greenhouses, which have been 

 painted with whitewash to reduce the intensity of the rays of light, have 



