THE RELATION OF HAIRY LEAF COVERINGS TO THE 

 RESISTANCE OF LEAVES TO TRANSPIRATION.*! 



J. D. Sayre. 

 INTRODUCTION. 



A great many of our common plants have hair-like structures 

 on their leaf surfaces. These epidermal hairs can readily be 

 divided into two classes; those that contain protoplasm, and 

 sometimes chlorophyll, which are called glandular hairs; and 

 those that are dead, and are filled with air. 



It is obvious that the first kind of hairs add to the evapo- 

 rating surface of the leaf, but there is some doubt as to the 

 effect which dead air-containing hairs have on the water loss 

 from leaves. The following paper deals only with the dead air- 

 containing hairs. There are several different forms of these 

 hairs: (a) Unicellular structures perpendicular to the leaf 

 surface; (b) One or more celled structures lying parallel with 

 the leaf surface; (c) Unbranched, many-celled structures, 

 extending more or less parallel to the leaf surface; (d) Many 

 celled structures, much branched, having the branches both 

 parallel and perpendicular to the leaf surface. 



The mullein plant {Verbascum thapsus) which was used in 

 the following experiments, is covered with hairs of the last 

 named type. Of all the plants found in this locality it has the 

 densest covering of hairs. The experimentation involved 

 the obtaining of continuous records of the common environ- 

 mental factors influencing plant processes, with records of 

 the water loss from the plants. 



HISTORICAL. 



Statements have been made by various authors as to the 

 relation of hairs to transpiration, but these assertions do not 

 seem to have been founded on experimentation. 



Warming (1) says: "It is evident that transpiring will be 

 very materially reduced when the transpiring surface is clothed 

 by air-containing bodies, in and between which the air is so 

 firmly lodged that its circulation is obstructed," 



* Thesis presented for Degree of Master of Science at the Ohio State 

 University, Columbus, Ohio. 



t Papers from the Department of Botany, Ohio State University. No. 116. 



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