THE AMERICAN NATURALIST 



[Vol. LV 



ing point size are definitely related to one another or 

 not. The organ most readily studied and most clearly 

 significant in such a problem is of course the leaf. 



The bean plant is evidently not well suited for such a 

 study, since its pith is rather irregular in outline and not 

 sharply delimited. The twigs and leaves of the rock ' 

 maple {Acer sacchanim), however, on which the writer 

 is carrying on some other work, have proven very satis- 

 factory for an investigation of this kind. The pith in 

 this species is approximately circular in cross section 

 and is sharply delimited, and the leaves are of uniform 

 and fairly considerable thickness. 



A series of twigs collected during the summer from a 

 single tree were studied. The area of the blade was de- 

 termined by tracing its outline on standard weight paper, 

 cutting this out and weighing the cut-out. Blade thickness 

 was measured by a micrometer caliper at two points away 

 from the main veins and situated symmetrically on oppo- 

 site sides of the midrib, the average of the two measure- 

 ments being taken. The product of area X thickness of 

 course gives us the blade volume. To determine pith area 

 a cross section was cut at the middle of the intornode,, 

 the pith diameter measured in two directions at right 

 angles to each other by a micrometer stage, the results 

 averaged and the area computed therefrom. 



Total blade volume (the sum of the volumes of the two 

 blades borne at a given node) was correlated with the 

 cross-sectional area of the pith of the internode below for 

 over 100 leaf pairs from this tree, taken from all parts 

 of its crown. The results are shown in Table VI. It is 

 quite evident from the size of the correlation coefficient' 

 (+.807± .024) that there is an unquestionable relation- 

 ship between leaf size and pitli area, the size of the leaf 

 being governed pretty largely by the stoutness of that 

 portion of the twig from which it springs. It would seem, 

 therefore, that the size ivlatioiisliips hctwccn the struc- 

 tures laid down by the tcniiinal inci-istciii jxTsist ns these 

 structures develop to maturity, these relative sizes re- 



