RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 27 



meadows of an open woodland formation in Skabholm. The 

 physical and chemical nature of the soil in the various locations was 

 carefully determined but no measurements of the amount of soil- 

 water seem to have been made. Of particular interest, in view of 

 the methods of the present paper, are the exact measurements of 

 light and their direct connection with assimilation, transpiration and 

 leaf structure. The conclusions in this respect are the following: 

 plants in the leafless thickets of the springtime assimilate as vigor- 

 ously as those in the sunny meadow ; reduced light decreases assimi- 

 lation even to the point of the complete absence of starch formation ; 

 plants which mature in continually decreasing but not very weak 

 light, have a less completely developed assimilative tissue than those 

 plants which obtain a great deal of light in the spring but are 

 deeply shaded during the summer; shade-plants transpire less than 

 sun-plants, and of the latter those with well developed palisade 

 transpire more than those with less differentiation in the leaf 

 structure. 



III. PHYSICAL FACTORS 



The physical factors of a habitat are either climatic or edaphic. 

 The former are those of the atmosphere, e. g. } light, temperature 

 and humidity, while the latter are connected with the soil, vis., water, 

 chemical and physical composition, and temperature. Of climatic 

 factors light is by far the most important in its relation to the plant. 

 Some of the light rays are reflected from the surface of the leaf 

 and thus rendered ineffective. The waxy coating of some leaves 

 serves the purpose of increasing the amount of reflected light and 

 so preventing over-illumination. Other of the light rays are trans- 

 mitted, and it is not known what effect they may have in trans- 

 mission. Effective light rays are those which are absorbed either 

 directly from the air or upon being reflected from the surface of 

 the soil. The latter, in connection with the heat reflected, especially 

 from light-colored soils, is a considerable factor in leaf structure. 

 Absorbed light acts upon the leaf through the assimilative function. 

 High light values, by increasing assimilation, cause an increase in 

 the assimilative parts, such as chloroplasts and palisade cells, 

 whereas low light values have the opposite effect. Humidity 

 affects leaf structure through increase or decrease of evaporation, 

 while temperature acts through increase or decrease of humidity. 



Of edaphic factors, the available water-content of the soil is by 



