PLANT COMPANIONSHIP IN THE FOREST. 171
two trees I have mentioned as having purple-leaved varieties belong to the
Cupuliferae, as also does the Hornbeam (Figs. 1 and 2) ; and this order,
together with the Salicineae and the Coniferae (three groups which
include a large proportion of our forest trees), are characterized by the
absence of hairs on their roots, the place of such hairs being apparently-
supplied, or more than supplied, by the presence of a mass of fungal
threads or hyphae known by the provisional name mycorhiza. It is
noteworthy that the "richer" the soil, i.e. the more humus it contains, the
more abundant is this mycorhiza. In the case of these trees it may be the
mycelium or spawn of the truffle-like genus Elaphomyces, which is, I am
informed, collected on this Forest as an esculent by the German inhabitants of
the East-end. In their case too, it is mainly what is termed ectotropic, i.e., not
penetrating the roots, whilst in the Heath tribe Ericaceae and Orchidaceae, in
which a similar growth occurs, it is endotropic, actually penetrating the roots
like a parasitic mould. Contrary, however, to what might have been expected,
Fig. 1. Mycorhiza or projecting Fig. 2. Part of cortex of root of horn-
ends of hyphae on young root of beam, with mycelial mantle. (Transverse
hornbeam. section).
it has been experimentally demonstrated by Frank that the presence of
mycorhiza is beneficial to the flowering-plants on which it occurs ; and it
would seem that it may absorb and transmit to its host-plant some of the
carbon-compounds present in humus, thus to some extent rendering chloro-
phyll unnecessary, and perhaps explaining the manurial value of humus which
has been a fruitful subject of debate among physiologists from the time of
Liebig.
As to nitrogen, it is well-known that the ordinary higher plants do not
assimilate this gaseous element in the free state in which it exists in the
atmosphere ; and it has been shown that they do not as a rule take it in
as soluble ammonium-compounds, as might well be supposed, but as nitrates.
The phenomenon of nitrification, or the formation of nitrates in the soil, was
found to present all the features which are distinctive of those processes known
as fermentations in which a micro-organism is present, being, for instance,
inhibited by the use of antiseptics. It was then traced to the action of
various Bacteria belonging to the genus Rhizobium, which are found to convert