ee SEARS—TARAXACUM 445 
of the figure and description of Leontodon latilobum given in BRITTON 
and Brown’s Iilustrated flora, 2d ed. It is clearly a juvenile rosette 
and hence not a valid type for comparison. Possibly it is flowering 
in the juvenile condition because of an autumnal start followed by 
a winter rest, as previously described. It certainly may be a 
legitimate distinct species, and so may a great many others, but it is 
not too much to suggest that taraxacologists have been ignoring 
grave sources of error, and to insist that future diagnoses be placed 
upon a satisfactory physiological basis. 
Summary 
1. Senescent and rejuvenescent change in leaf form is a prime 
factor in producing variations in Taraxacum vulgare and T. laevigatum. 
2. Senescence produces a steadily increasing degree of dis- 
section, and frequently of hairiness; rejuvenescence restores the 
unsegmented juvenile seedling leaf form. 
3. These changes are independent of changes in (a) absolute 
leaf area, (b) total xylem duct capacity in successive leaves, (c) mean 
xylem duct capacity in successive leaves, (d) ratio of total capacity 
to leaf area, and (e) number of vein islets per unit area of leaf. 
4. Senescence, however, is accompanied by a marked increase 
in carbohydrate-nitrogen ratio and rejuvenescence by its abrupt 
decrease. 
5. Environmental factors are of secondary importance in so 
far as studied. Moist atmosphere causes elongation of leaves. 
Time of seeding may govern time of flowering and rejuvenation; 
this in turn may affect degree of dissection in plants at flowering 
time. 
6. Pure stands of distinctive variants in certain cases may be 
explained by environmental control of seed germination time. 
7. The majority of so-called specific characters in Taraxacum 
are subject to extreme fluctuation. 
8. These fluctuations in other than leaf characters are in some 
cases due to senescent change, in others to environmental factors, 
but mostly to causes not known. 
9. Such fluctuations, together with senescent and ecological 
_ changes in leaf form, are potent enough when working upon the 
