378 Holm . — On Obolaria virginica. 
in regard to the number of strata and the size of the cells ; 
moreover, the greater part of the bark-cells will be found to 
be filled with glomerules of fungal hyphae, showing that the 
root has become a mycorhiza. This form of root was first 
described by Frank, and, so far. has been found only in a few 
Gentianaceae, viz. the saprophytic genera Voyria and Voy- 
riella. According to Warming, mycorhizae are characteristic 
of several Ericaceae and Orchidaceae which thrive only in 
a rich humus. In these, as in Obolaria , the mycelium is 
found exclusively in the bark-parenchyma. The mycorhizae 
also occur in many trees, and Sarauw has given a mono- 
graphic study of them. 
In Obolaria the swelling of the roots is due to the presence 
of these fungal hyphae, which undoubtedly cause a certain 
irritation, not only of the bark, but also of the tissues inside 
of the bark. The cells of the endodermis (Figs. 15 and 16) 
show numerous divisions, both tangential and radial. The 
peri cambium shows similar divisions and forms several strata, 
while the inner mass of the root-cylinder, the hadrome and 
leptome, undergoes a considerable disturbance as regards the 
arrangement of its tissues. The groups of leptome are 
imbedded in a huge mass of cambial layers, which surround 
a number of vessels, constituting the innermost part of the 
root-cylinder. As regards the arrangement of tissues, there 
is in this a marked difference from that observed in the young 
roots, as described above. 
In the old root there is nothing to indicate its originally 
diarchic type. The interior tissues, from the bark to the 
hadrome, undergo an increase so sudden and irregular that 
the position of the various elements is changed and an ab- 
normal development of cambial tissue takes place. This is 
all the more peculiar when it is remembered that our plant 
is annual. Had the anatomical structure of the root been 
known, there would have been good reason to suspect Obolaria 
to be saprophytic ; but as the roots do not show any haustoria 
there was no reason to suppose it to be a parasite. 
In discussing the question as to whether our plant is really 
