Sources of Nitrogen in Plants. 327 
grow out into the soil parallel to one another, and form 
compound strands in every essential respect like the Rhizo- 
morphs (of Agaricus melleus for instance) ; from such strands 
free hyphae may radiate out into the soil in their turn again. 
Anastomoses and ramifications of the most varied kind may 
take place in the surrounding soil, and the regions where 
Truffles grow have the soil permeated with such systems. 
From the study of longitudinal sections, &c., Frank con- 
cludes that the development of the Mycorhiza is some- 
what as follows. Since the mycelium closely invests the 
whole root-tip, it must elongate coincidently with the root : 
as matter of fact the hyphae covering the root-cap are 
thinner, and show every sign of growth, both by elongation 
of the existing hyphae, and by the interpolation of new 
branches between those already formed. The ‘ endophytic 
hyphae’ do not develope until the growing apex has passed 
out of the stage of elongation ; hence no organic connection 
between fungus and root is formed at the apex. The com- 
plete covering of the apex seems to be causally connected 
with the very feeble development of root-cap cells — in other 
respects the root (that of Carpinus is figured) conforms to 
the common type for Dicotyledons ; probably, on the one 
hand, the pressure prevents the fuller development of root-cap 
cells, and, on the other, the fungus-web has acquired the 
protective function of a true root-cap. 
Although the first stages of germination of the tree are 
passed through without the appearance of the fungus on 
the radicle, the lateral rootlets are usually soon attacked. 
The hornbeam is attacked very early, the young oak may 
remain a year or two free from mycelium. 
In contrast to the roots of plants cultivated without the 
fungus, the Mycorhiza is shorter and thicker — the number 
of layers in the plerome and periblem increase : the tendency 
to branch is also increased, and the lateral roots emerge 
at points closer together, on account of the slow growth 
in length. These peculiarities give the Mycorhiza a £ coral- 
like’ shape. As regards endogenous origin, monopodia! 
