339 
and its Mycorhiza . 
between the cytoplasm of the host and that of the Fungus, 
but there never was any indication of such a connexion, 
though each hypha is externally coated from first to last 
by a sheath of cytoplasm. 
The differentiation of the definite layers of the thallus is not 
directly caused by the mycorhizal organism , for the distinction 
of the various layers (sheath, exocortex, limiting layer, rest of 
cortex, and stele) appears in young portions of the apex before 
the hyphae have reached the cells of that region. 
I did not trace the mode of entrance of the hyphae into 
the sheath-cells, but in all the deeper infected tissue the 
Fungus enters the cell as a single slender hypha zvhich at once 
grows directly towards the nucleus of the host-cell (Figs. 6, 16). 
That it is not the nucleus which travels towards the hypha is 
shown by the fact that the nucleus preserves its original 
central position before and for some time after the hypha 
has reached it. 
The later career of the mycelium represented by this single 
hypha depends on the position of the host-cell. As the 
subsequent changes are most clearly traceable in the inner 
layers of the cortex (mediocortex) an account will be given 
first of the development of the mycorhizal Fungus in this 
region. 
Development and Fate of the My celia in the Mediocortex 
(Figs. 6-1 1 ). 
With the penetration of the embryonic hypha, the starch- 
grains of the cell disappear never to return, or to reappear 
only when the hyphae are dead or dying. When treated with 
Gram’s method (methyl violet) and cleared with oil of cloves 
containing eosin, the protoplasm of the hypha stains blue, 
and its numerous minute nuclei become deep blue ; the 
cytoplasm of the cell and the main body of the cell-nucleus 
colour pink, but the nucleolus and some large granules in the 
nucleus stain deep blue. The hypha having reached the 
nucleus commences to form a local pear-like or oval swelling 
against the nucleus. This young bladder is really intercalary, 
