PECULIARITIES IN GROWTH OF UAWTIiOKN-TKEE. 369 
branch leaves the stem. This evident dependence of the rib upon the 
branch with whose under-surface it is continuous midit at first si^-ht 
seem to give countenance to the exploded view which regards the 
annual layers of wood as a longitudinal growth sent down by the 
leaves and buds] but, it is quite consistent with the now generally 
accepted view that — not the wood itself, but the elaborated sap out of 
which the wood is formed, is that which is sent down by the leaves. 
On this view w^e must^ it would seem, believe that the elaborated sap 
has descended luost freely along that vertical course marked by the 
largest amount of woody growth. The real difficulty to explain Is, why 
it should be restricted to this course, and why it should not, as in the 
Oat or the Ash, be diffused more uniformly around the stem and con- 
tribute to the formation of complete annual rings of wood. It is con- 
ceivable that there is some peculiarity of structure, in consequence of 
which the tissues transmit fluids more readily in a vertical than in a 
lateral direction ; but this can only be detei'mined by careful experi- 
ment and the aid of the microscope. 
This peculiarity in the growth of the Haw^thom has, I think, an 
additional interest from its approach to some of the anomalous struc- 
tures seen in tropical trees and shrubs. We can scarcely look, for 
instance, at Lindley's figure of Euonymus tingens, Introduct. 1, p. 214, 
without recognizing in it something like the Hawthorn. "The forma- 
tion of vertical plates or ribs (or, when old, buttresses) in Exogens'* 
is said to be far from uncommon in tropical countries [I have ob- 
served it in Heritkra Utoralis, Inocarpns eduUs, and many species of 
FicuSy — Ed.], but I am not aware that any one has pointed out that 
we have something of the kind in our own country. 
Nor is the Hawthorn the only British tree on whose stem these ribs 
or projections are formed, although I believe it is the only one in which 
the heartwood decays and sets free the ribs as independent segmental 
stems; The Yew-tree may perhaps be an exception, but of this I can- 
not now be sure, though I am certain that in it the ribbed or fluted 
character of stem is sometimes very manifest. I have also noticed it 
in the Lombardy Poplar, and in very old examples of the Small-leaved 
or Wild Lime {Tilia par vif alia). In other trees also, where there is 
no actual rib, we may notice a tendency towards the same kind of 
structure in the sub-angular form assumed by the stem. In the case 
of the Birch-tree there are very commonly two ribs running downward 
