106 ROBERT TRACY JACKSON ON 



leaves are opposite, simple, oval, with acuminate tips and long petioles. Several alter- 

 nating pairs (five or six) of simple opposite leaves appear, all having the same outline, 

 excepting that occasional abnormal leaves are blunter distalh' or even rounded and dis- 

 tally emarginate. These variations in the cases noted usually occur in both leaves of the 

 pair. The first compound leaves, as shown in an older seedling (PL 18, fig. 46), are tri- 

 foliolate, the distal leaflet resembling the earlier simple leaves; the lateral leaflets are 

 similar but sessile. Gradually later added leaves become more differentiated by the addi- 

 tion of lateral leaflets proximally, until there are from seven to nine leaflets, which are 

 characteristic of the species. During later growth, petiolules develop on the lateral leaf- 

 lets, until they are comparatively long, as in tlie adult (PI. 18, fig, 48). 



In the adult White Ash the leaves (PI. 18, fig. 48) are large, and the terminal leaflet 

 closely resembles the simple nepionic leaves of the seedling. The White Ash suckers 

 quite freely from roots near the base of the tree or from the stump when a tree has been 

 cut down. Such suckers repeat very closely the form of seedlings. One of a number of 

 suckers from the base of a tree some forty feet high is represented in PI. 18, fig. 47. 

 The first leaves are simple and alternate, as is frequent in suckers of the ash, although 

 they are often opposite, as in seedlings. The first simple leaves also are often rounded 

 distally instead of acuminate as in seedlings. These and other variations are more fre- 

 quent in suckers from large trees than from small ones. After more or less simple leaves, 

 usually several, the compovmd leaves appear. The first compound leaves are usually tri- 

 foliolate as in seedlings, in later leaves lateral leaflets are progressively added proximally, 

 as in the growing seedling, also lateral leaflets are sessile as in seedlings. The last of 

 the simple leaves and the compound leaves (PI. 18, fig. 47) are quite strongly serrate, a 

 feature which is less marked or absent in seedlings. In suckers from stumps of large 

 trees that have been cut down, the first leaves are often compound, being trifoliolate or 

 even five-foliate ; nevertheless suckers with simple leaves at their base may be considered 

 the typical form. The only important differences noted between typical suckers and 

 seedlings are that leaves are very commonly alternate in suckers and that they undergo 

 frequently an accelerated development, acquiring adult characters earlier than seedlings. 

 To this may be added considerable variability in the outline of leaves in suckers. 



In growths thrown out late in the season from any part of the tree, it is noticeable 

 that the lateral leaflets are frequently sessile, in this resembling the character of nepionic 

 leaves. In the adult White Ash occasionally leaves of three or five leaflets, or rarely 

 simple leaves, are found instead of the typical number. Such primitive leaves may fairly 

 be considered as cases of arrested development, in which during the ontogenesis of the 

 leaf the later added characters were skipped. It is to be noted, as observed in many 

 other caseSj that it is the proximal, or last added, portion which is repressed. 



