Maryland Geological Survey 485 



of the leaf for some distance; the midnerve above the base at different 

 points sends off minor nerves, which curve upwards and fork; ultimate 

 nervation not seen." — Fontaine, 1890. 



While the outline of this leaf does not exactlj' conform to any modern 

 bilobed Sassafras leaf known to the writer, it is nearer the latter than 

 it is to any other leaf. The right-angled sinus with straight sides and 

 running to a point is also a feature not seen in the modern leaf. In. the 

 latter, when the sinus runs to a point it is narrow and deep; and when 

 it forms a right angle it is curved and the resulting lobe is generally 

 obtuse and but slightly produced. We have characters which ally this 

 ancient leaf to Sassafras in the decurrent base ; the subopposite primaries, 

 as they usually are in the bilobed leaves of the existing Sassafras: tlie 

 position of the secondaries; and especially in the secondary running to 

 the sinus, a feature we would hardly expect to find in so primitive a 

 leaf. We would consider the bilobed leaf as a more ancient tj^pe than 

 the trilobed form, and removed from the ancestral simple • leaf by a 

 series beginning with leaves with but a slight depression marking the 

 position of the future sinus, and a slightly produced obtuse lobe, through 

 forms partially paralleled in. the modem, leaf, in which these features 

 were more and more emphasized. Just why the leaf became lobed is 

 largely conjectural. The primaries form a more acute angle with the 

 midrib than do the secondaries, especially in the simple leaves; they are 

 the first and largest arteries branching from the midrib; in the growing 

 leaf they are carried upward, while new laterals are added toward the 

 tip of the blade. It may be that the diagonal position of the leaves in 

 the bud causes pressure at that particular lateral part near the tip of the 

 primary, causing more or less atrophy of that part of the blade. This 

 tendency once inaugurated the rest is simple, for those portions of the 

 leaf at the tips of the primaries would have nearly all their leaf-forming 

 energy expended in increasing the length of the lobes ; possibly especially 

 good environment was a factor in the original location, as witness the 

 great development of the lateral portions of the leaf blade in the five- 

 lobed forms occurring in rich soil. The Potomac species under discus- 

 sion bears some resemblance to certain species referred to Sterculia, as 



