Observations upon some Noteworthy Leaf Variations. 293 



the foliage is always larger than common, and reaches a maxi- 

 mum diameter of 24 cm. with an area 491 sq. cm. While it 

 is obvious that the primary cause of this very wide variation is 

 due to great vigor in growth as induced by highly favorable 

 conditions of nutrition, it is no doubt influenced by the shade 

 in which such plants grow in most cases. The general result, 

 however, is to produce leaves, the extreme variation in which 

 is in the ratio of 1 : 8.90 ; while the ratio of the average leaf 

 to the extreme of development on undershrubs is as 1 : 5.24. 

 A recapitulation of these facts would give the following : — 



TlLIA AMERICANA. 



Comparative leaf areas. 



No. 



Area in sq. cm 



Ratio of areas. 



Remarks. 



1 



55.18 



1:0.58 





2 



93.75 



1:1 



Leaf of average size for most trees 

 taken as unity. 



3 



276.47 



:12.94 



The largest foliage of mature trees. 



4 



491.25 



1:5.24 



The largest foliage of undershrubs. 



Extreme variation 



1:8.90 



We may now consider the foregoing facts in relation to 

 their bearing upon palasontological evidence, and ascertain if 

 possible, the causes for such variations. 



Prof. Ward says : " It is a fact well known to botanists 

 that, in oaks and many other trees, only the leaves on fruit 

 bearing branches can be depended upon for determination 

 of species," {op. at. 41). While this is no doubt true in 

 particular cases, it would be unfortunate were it to be 

 adopted as a working rule of general application, since note- 

 worthy instances of frequent occurrence readily suggest 

 themselves to one as affording evidence in direct opposition to 

 such a view, and the examples now under consideration are of 

 that nature. In the case of Spiraea, the leaves arising upon 

 the fertile branches are always greatly reduced in size and 

 much altered in form Their average area is less than one- 



