210 ILLINOIS STATE ACADEMY OF SCIENCE 



areas. The area in square centimeters of the end leaf- 

 lets from these fonr representative plots were: 



Lot B, 63 



Lot D, 36 



Lot F, 17 



Lot G, 12 

 These areas, as you see, conform with the types of their 

 environments. 



This has not proven true for the thickness of the 

 leaves. When sections were made and measured, there 

 seemed to be almost no relation between xerophytism and 

 the thickness of the leaf. In the diagram, the leaf sec- 

 tions were arranged in the same order as the leaf areas 

 in the preceding diagram. As you can see, the meso- 

 phytic leaves may be as thick as the xerophytic, or even 

 thicker in some instances. 



The last and most important contrast is in the micro- 

 scopic study of the sections. Though it seems impos- 

 sible to tell by the thickness of the leaves whether they 

 are mesophytic or xerophytic — when the general com- 

 pactness of the tissue is considered, the difference is very 

 striking. Cross sections were drawn on graph paper 

 in order to determine the relation between the total area 

 of the leaf section and the part of that area occupied by 

 cells. In this way a coefficient of compactness could be 

 determined. This work is not yet finished and I am not 

 ready to give a conclusive statement, but so far as this 

 has been considered the coefficient of compactness is : 



Lot B, 61 



Lot D, 74 



Lot F, 87 



Lot G, 89 

 This conforms with the leaf area and type of environ- 

 ment. 



Considering the individual elements, as seen in the 

 cross section of the leaf, the greatest variation is in the 

 upper epidermis and the spongy tissue. The variations 

 in the upper epidermis are marked, the epidermis being 

 much thicker when the leaves were growing in xerophytic 

 regions. Some species of plants have variation in the 

 number of the rows of palisade cells when growing in 



