180 THE ESSEX NATURALIST. 
of these the hyphe 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 
