Sources of Nitrogen in Plants. 335 
measurement of thickness includes the mycelial envelope as 
well as the rootlet proper : a vascular bundle of a few 
elements runs down the axis. The fungus was on the outside 
only. No such Mycorhiza could be found on a specimen 
of Pinus Pinaster in the Botanic Garden at Berlin. 
As regards the Mycorhiza on the roots of Ericaceae (4) 
it appears that one or two observers had already found here 
and there instances of association, more or less regular, 
between hyphae and roots. 
In the Ericaceae the simpler roots may consist only of a 
few tracheides and sieve-tubes surrounded by relatively huge 
epidermal cells, each of which may occupy one-sixth of the 
periphery. There are no root-hairs. Each of these very large 
epidermal cells is filled with a dense complex of extremely fine, 
interwoven fungus-hyphae : these are so densely crowded that 
they form a sort of pseudo-parenchyma. ‘ In most cases 
these fungus elements are so fine, that one may be in doubt 
whether this intracellular mass is to be explained as a fungoid 
pseudo-parenchyma.’ Frank has no doubt of this, however, 
since he can trace the finest hyphae from certain coarser ones 
which pass into the cells from the outside. The growing- 
point of the root of Andromeda polifolia is curiously reduced, 
and the author finds that it possesses an apical cell, triangular 
in surface view, from the segments of which the other tissues 
proceed. The dermatogen runs all round : the root-cap is 
reduced to two or three small loose cells ; and the plerome 
cylinder is also extremely simple. The fungus fills the cells 
of the dermatogen up to the extreme apex, and the fine 
mesh-work alluded to above can be detected in all but the 
youngest cells. 
On the surface of the root are loose hyphae, as a rule, and 
sometimes they cover the root rather thickly; even when 
these outer hyphae are absent, the intracellular fungus is 
present. In Vaccinium Oxycoccus the author traced the con- 
nection between the thicker hyphae outside and the finer ones 
in the epidermal cells, and also found hyphae running in the 
rather thick cell-walls. In some cases the superficial hyphae 
