34i 
and its Mycorhiza. 
an adjoining cell which lies nearer the apex or is situated 
nearer the centre of the root. This young hypha again 
repeats the same history as in the preceding cell. Hence in the 
cells of the mediocortex, the youngest bladder is always to be 
found lying against the nucleus, although the nucleus does 
not preserve its original central position. In an old cell in 
which all the bladders are dead, the main mass of the cyto- 
plasm, as also the nucleus and its bubble-like nucleolus, stain 
pink, but minute granules in the nucleus still stain blue, when 
treated with Gram’s method as already described. No hyphae 
enter the raphide-mucilage sacs. 
The My celia in the Sheath-layers. 
The hyphae in the sheath behave very differently. The 
hypha runs straight through the middle of the cell in 
a longitudinal direction. The nucleus of the host-cell may 
be often seen lying close against this hypha. The hypha 
undergoes no local hypertrophies ; it increases evenly in 
volume, so that an old hypha is merely stouter than a young 
one, and its contents do not stain so intensely blue. The 
mycelium in these sheath-layers is very long-lived, so that 
in old parts of the thallus it is still living and protoplasmic. 
There is at most a feeble deposit of excreta as represented by 
a more or less faint yellow colouration and minute yellow 
granular clumps in the protoplasm of the old hyphae. 
The Mycelium of the Exocortical Layer (Figs. 12-1 5). 
The hyphae of the characteristic exocortical layer are 
intermediate in behaviour between those of the sheath and 
those of the mediocortex. The slender hypha on reaching 
the nucleus does not at first undergo considerable local 
hypertrophy ; it grows rapidly and coils about in the cell, 
and the older parts increase at first tolerably regularly in 
volume. It seems probable that the youngest part of the 
hypha always tends to follow the host’s nucleus. For in 
sections one often sees this youngest part close to the nucleus, 
and frequently fails to find a young slender hypha distant 
