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igi7l STOBER— WINTER AND SUMMER LEAVES 91 



am 



)le, regardless of any special protective structures. The stem 

 leaves, on the other hand, are usually borne some distance above the 

 ground and exposed to greater intensity of light, stronger winds, 

 and greater extremes in temperature, humidity, and transpiration. 



Method of study 



Most of the plants used in this comparative study were collected 

 in the region about Chicago; the remainder, in eastern Penn- 

 sylvania. In order to simplify matters, all winter leaves, whether 



produced in 



runners, or on basal 



designated 



be designated as cauline or stem leaves. 



Leaves for study were killed, fixed, and preserved in a 4 per 

 cent solution of formaldehyde in 50 per cent alcohol. Delafield's 

 haematoxylin was used as a general staining reagent. Sections 

 were also treated with chloriodide of zinc, the cellulose wall 

 turning blue, while the cuticle and cutinized portions of the 

 epidermal wall turned yellow\ Alcannin tincture 





I of zinc. 



yellow. Alcannin tincture imparts a pink 

 much slower in its action than chloriodide 



Unless otherwise specified, all observations were made on the 

 middle of the leaf, from the midrib to the margin. All observations 

 and measurements were made with f , |, and T V in. (oil immersion) 

 objectives, and with a 1 in. micrometer eyepiece with divisions of 

 0.1 mm. Camera lucida drawings were made of portions of the 

 epidermis for measurement and comparison of epidermal cells and 

 stomata. Chloral hydrate was used as a clearing agent for leaves 

 to facilitate the study of air spaces and packing of mesophyll tis- 

 sues. Most measurements and counts represent an average of 

 5-20, depending upon the degree of variability of the objects meas- 

 ured. Measurements are expressed in microns, and counts rep- 

 resent the number in the field under low or high power, which is 





indicated in each case. 



same species ( 



cases where the plants could readily be secured) were studied at 

 different times and the results compared. These results varied 

 only slightly when the plants came from the same habitat, but 

 usually differed considerably in plants from different habitats. 



