and its Mycorhiza. 333 
send off straight branches which run obliquely outwards or 
obliquely inwards (Fig. 2). There are in addition minute, 
blunt, nearly solid protuberances on the sides of the hyphae : 
these are arrested branches. The hyphae are mostly unseptate 
inside the sheath, but some of the older hyphae are divided 
by transverse walls. They are continuous with septate 
hyphae outside the plants and also with the hyphae occupying 
the deeper layers of the organ. 
(2) Exocortex. — Within the innermost layer of sheath-cells 
a sudden and remarkable change in the tissue takes place. 
There succeeds a single unbroken and very regular layer of 
cells, with no intercellular spaces and having a quite different 
shape and strikingly different contents. These cells vary 
in form from narrow palisade-like cells elongated radially, 
to more or less square cells : but in any case they are much 
broader radially than the sheath-cells. They have thin 
cellulose-walls. But in their contents they form the greatest 
contrast to the sheath. Every cell is almost filled with a 
coiled mycelium consisting of swollen irregidar moniliform 
mycorhizal hyphae with densely staining protoplasm . The 
protoplasm of the host-cell coats these hyphae and lines the 
wall ; there is a conspicuous nucleus, and a relatively small 
amount of cell-sap. I pointed out that in the absorbing axes 
(root or stems) of holosaprophytic Orchids some of the outer 
tissue entertains living coiled mycorhizal mycelia, and that 
this tissue is sometimes differentiated even in the growing- 
point before the hyphae have reached it : to it I gave ten- 
tatively the name exocortex 1 , and the same term may be 
employed to describe this single layer in the absorbing organ 
of Thismia. Very rarely one of the cells of this layer in 
Thismia is changed into a raphide-mucilage cell. No starch 
is found in the mature cells. 
(3) Limiting Layer. — Within the exocortex a layer succeeds 
which again sharply contrasts with it. The cells are paren- 
chymatous, radially flattened, smaller than the preceding, 
1 Percy Groom, Contributions to the Knowledge of Monocotyledonous 
Saprophytes, Journ., Linn. Soc. read Dec. 20, 1894. 
A a 2 
