_Walnut_ (_Juglans regia_).--This tree and the common nut belong to a
widely different order from the foregoing fruits, and are therefore
here noticed. The walnut grows wild in the Caucasus and Himalaya, where
Dr. Hooker[743] found the fruit of full size, but "as hard as a
hickory-nut." In England the walnut presents considerable differences,
in the shape and size of the fruit, in the thickness of the husk, and
in the thinness of the shell; this latter quality has given rise to a
variety called the thin-shelled, which is valuable, but suffers from
the attacks of tom-tits.[744] The degree to which the kernel fills the
shell varies much. In France there is a variety called the Grape or
cluster-walnut, in which the nuts grow in "bunches of ten, fifteen, or
even twenty together." There is another variety which bears on the same
tree differently shaped leaves, like the heterophyllous hornbeam; this
tree is also remarkable from having pendulous branches, and bearing
elongated, large, thin-shelled nuts.[745] M. Cardan has minutely
described[746] some singular physiological peculiarities in the
June-leafing variety, which produces its leaves and flowers four or
five weeks later, and retains its leaves and fruit in the autumn much
longer, than the common varieties; {357} but in August is in exactly
the same state with them. These constitutional peculiarities are
strictly inherited. Lastly, walnut-trees, which are properly monoicous,
sometimes entirely fail to produce male flowers.[747]

_Nuts_ (_Corylus avellana_).--Most botanists rank all the varieties
under the same species, the common wild nut.[748] The husk, or
involucre, differs greatly, being extremely short in Barr's Spanish,
and extremely long in filberts, in which it is contracted so as to
prevent the nut falling out. This kind of husk also protects the nut
from birds, for titmice (_Parus_) have been observed[749] to pass over
filberts, and attack cobs and common nuts growing in the same orchard.
In the purple-filbert the husk is purple, and in the frizzled-filbert
it is curiously laciniated; in the red-filbert the pellicle of the
kernel is red. The shell is thick in some varieties, but is thin in
Cosford's-nut, and in one variety is of a bluish colour. The nut itself
differs much in size and shape, being ovate and compressed in filber