THE FUNGUS-ROOT (MYCORRHIZA). 187
England since these observations were commenced.
Mycorrhiza were unusually abundant during the months
of February and March 1923. Root-hairs of this root-
system are entirely suppressed.
(2) Seedling birches carry mycorrhiza during the first year
of growth, yet they lengthen as much as two feet during
that time. There are no apparent signs of injury to the
seedling by the fungus.
(3) Mycorrhiza become dry and shrivel within a short time,
(30 minutes or less) on being exposed to a dry atmos-
phere. They readily part with the water that has been
absorbed from the soil.
(4) The structure of the mycorrhiza, when the radially-
elongated cells are present, provides an effective
channel for water passing from the fungus-mantle to 
the central axis of the root.
(5) The thicker roots from which the filiform rootlets, that
bear mycorrhiza, arise, have well-developed secondary
wood, through which water passes readily to the stem.
The wood of these thicker roots is often markedly
excentric. This may mean little, but the fact is re-
corded for reference.
(6) The means for carrying on physiological processes do not
appear to be specially evolved for the benefit of the
fungus. Different fungi produce different mycorrhiza
upon the same tree at the same time. On the rootlets
of the birch there are frequently found two distinct
forms of fungus-root, one being brown and the other
whitish-grey.
At the end of May, when birches are normally in full foliage,
the leafless condition of the top branches of a large percentage
of the trees was a conspicuous feature of birch woods in 1922.
The appearance resembled a state that was common among
birches during the years 1899-1901. Throughout that period
birch trees in the Forest, and elsewhere in the South-eastern
counties, were killed in great numbers by a microscopic fungus
parasite, Melanconis stilbostoma (Fr.) Tul., which commences
its fatal entry at the extremity of the top branches, follows a
course between the cortex and the wood, and ultimately reaches
the stem, down which it passes rapidly. The wood is killed and