358 GENERAL I'LANT PATHOLOGY 



number of cells, as for example, the omission of one of the palisade 

 layers of the leaf. External factors are important in determining the 

 structure of the leaf tissue, for the leaf more than any other plant 

 organ is an index of the influence of climate. This fact is empha- 

 sized by a work entirely devoted to this subject and given the 

 appropriate title of " Phyllobiologie." There is a marked difference 

 in the thickness of beech leaves, for example, which have developed 

 under different environmental conditions, as I have proved satisfacto- 

 rily by the use of calipers and microscopic measurements, which show 

 an accurate coincidence. The thickness, or thinness, of such a leaf de- 

 pends essentially on the number of rows of cells. The thickest leaves 

 with the largest number of palisade layers which I have studied, grew 

 in the bright sunlight in exposed places along the edge of a salt marsh 

 at Cold Spring Harbor, Long Island. Sun leaves back from the influ- 

 ence of salt water were thinner and broader, while those growing in 

 the dense shade of the forest in an inland situation near Philadelphia 

 were the broadest and thinnest of all. Not only was the mesophyll 

 modified in these leaves, but a marked difference was found in the shape 

 of the epidermal cells in the sun and shade leaves. 



The number of cells which arise from the cambial layer suffers a 

 marked diminution in trees which grow under unfavorable climatic life 

 conditions. Drought, strong winds, pressure, unfavorable light and 

 nutrition are disturbing factors. Growth activity of the cambium may 

 cease entirely, if these factors become too intensive. Huntington has 

 proved abundantly by his study of yellow pines of New Mexico and 

 the big trees of California that climatic cycles of wet and arid conditions 

 in the past history of North America can be determined from a study of 

 the size and character of the annual rings due to the cambial activity of 

 those trees, and he has plotted curves showing this relationship for a 

 period approximately 3500 years in the case of the big tree. Sequoia 

 gigantea} 



B. Size of Cells. — The size of cells must be considered also in dis- 

 cussing the phenomena of hypoplasia. Abnormally small cells may 

 be produced in different ways: A fresh division of the cells may take 

 place before the cells have reached the average size which they as- 

 sume under normal conditions. Klebs recites a case where he culti- 



1 Huntington Ellsworth: The Climatic Factor. Publ. 192, Carnegie Institu- 

 tion of Washington, 1914: 153. 



