1909] BRIEFER ARTICLES 461 
It is undoubtedly a fact that the great majority of woody dicotyledons 
have leaves which when freely exposed to the sun are concave on the upper 
surface and that this concavity usually lessens or disappears in the case of 
much-shaded leaves on the same plant. WIESNER’s conclusion that this 
conformation is due to the effects of powerful illumination seems on the 
whole to be plausible. It is not safe, however, to assume, as he does, that 
we have here an undoubted case of protective adaptation, to ward off the 
injurious effect upon chlorophyll of excessive insolation. In order to prove 
this it would be necessary to explain many real or apparent exceptions to 
the assumption that greater concavity should go with greater illumination. 
To cite the first instances that occur: Acer Negundo is hardly, if at all, 
more exposed to excessive sunlight in its usual habitats than is A. sacchari- 
num, and yet the former has trough-like leaflets, while the leaves of the 
latter are nearly flat. The Japanese Ligustrum Ibota comes from a region 
not enough more subject to excessive sunlight than that of the European 
L. vulgare to account for the great difference in the flatness of the leaves of 
the two species. Vinca rosea, a West Indian weed, might be expected to 
have leaves much more concave than those of the European V. minor, 
but the reverse is actually the case. Occasionally leaves growing in shade 
may be decidedly more concave than those growing under greater illumina- 
tion, as I have noted in one case of Kalmia angustifolia. Finally, it is 
difficult to explain on any theory of utility of concave leaf surfaces in protect- 
ing chlorophyll the fact that the angular values obtained for leaves of the 
same individual, grown under similar conditions, vary somuch. The angles 
given in the preceding table for each species were in many instances obtained 
from leaves which apparently received almost identical illumination, since 
they grew fairly near together and were sometimes almost in contact 
with each other. Yet, as will be observed, the angles for sun leaves some- 
times varied in a ratio as high as six to one. Equally difficult to explain is 
the fact that in some genera, e. g., Prunus, Pyrus, Salix, the whole tree 
may show little difference in the flatness of the leaves, all being angled 
whether they grow in sun or shade. 
Instead of trying, as WIESNER has done, to establish the critical intensity 
of illumination at which leaves of a given species cease to be panphotometric 
and become euphotometric, it would seem to the writer better worth while 
to look for some relation (always on the same individual) between illumina- 
tion and the average amount of angular divergence of the halves of the 
leaf, basing all statements on a large number of measurements.—JosEPH 
Y. BercEn, Cambridge, Mass. 
