180
THE ESSEX NATURALIST.
of these the hyphae often force a passage between the cortical:
cells and sometimes enter them.
Third Period 1894-1904. During this time the most im-
portant papers were mainly of two kinds, systematic and cyto-
logical: writers treated almost entirely of the endotropic forms
and chiefly of those associated with the roots of orchids. Numer-
ous theories were advanced respecting the relation between the
flowering plant and the fungus. The reason for investigation
being centred upon endotropic mycorrhiza is not surprising,
it was associated with plants that were highly valued and that
were, at that time, most difficult to raise from seed.
From 1904 onwards the number of writers increased rapidly.
Papers on endotropic forms were still the more numerous but
ectotropic mycorrhiza were not entirely neglected.
It is with the ectotropic fungus-root, and particularly with
that of the birch-tree, that I have been interested for the past
three years.
Mycorrhiza have been reported as occurring on the following
trees, shrubs and ground plants of our woods and forests, viz.:—
Oak, beech, hornbeam, birch, aspen, ling, whortle-berry, spurge
laurel, arum, Paris and bluebell. Those of forest trees are mostly
ectotropic. Birch mycorrhiza are remarkably abundant, espec-
ially on the roots of trees that are growing on a light soil with a
top layer of decaying leaves. In most cases that I have examined
there were very few rootlets that were not fungus-roots, and
yet birch is frequently omitted from the list of trees that bear
mycorrhiza. This is owing to the fact that Frank in his first
paper, April 1885, included the following paragraph "It is not
superfluous to mention that numerous other plants growing in
woods were examined, herbs, shrubs, trees, but birches, alders,
ashes, elms, were all devoid of the fungus." In a second paper,
however, in answer to criticisms, published November 1885,
Frank includes the birch among the trees bearing fungus-root.
This paper has been evidently over-looked by writers who omit
birch from their lists of forest trees bearing mycorrhiza.
At the time of writing, March 1923, one is able, on removing
a thin superficial layer of decaying leaves, to expose under birches
a network of root-fibres, of so fine a mesh that it is hardly possible
to put the end of an ordinary lead pencil between the meshes
without touching one of the boundaries. Mycorrhiza are