8 MacDougal. — Symbiotic Saprophytism. 
considered simply as a neutral bond between the other 
symbionts, and the symbiosis as dual and overlapping. 
The epidermis is composed of elements elongated parallel 
to the axis of the root, with the radial walls variously folded 
and curved. The walls are slightly suberized, and the outer 
membrane of many of them is extended into root-hairs, also 
suberized. The root-hairs are seen to be traversed by pale, 
highly refractive hyphae, which extend from the cortical 
cells through the hypoderm into the epidermal cells, and 
out through the hairs into the soil. The epidermis is 
penetrated by the hyphae from the mycorhiza of trees. The 
hyphae traverse the epidermis in a longitudinal direction, 
sending off lateral branches which show a few convolutions 
inside some of the cells, or penetrate one or two walls 
tangentially, in one or two instances into the exoderm. In 
the instances in which the tree-mycorhiza was applied 
directly to the root, the outer walls had disappeared, and 
the layer was replaced by an entangled mass of the Fungus. 
In any given transverse section the Fungus-layer might 
include one-fifth of the circumference, while nearly all the 
epidermal cells were penetrated more or less through the 
lateral walls by the main hyphae or its branches. The exo- 
derm consists of thin-walled rectangular cells in both cross 
and longitudinal section. The average length is equal to 
one and one-half or twice the short diameter. The proto- 
plasmic content of both the outer layers is very spare. 
Numbers of the exodermal cells are almost cubical, and 
contain a large proportion of protoplasm and deeply-staining 
nuclei ; such cells are penetrated by hyphae passing from the 
cortex to the epidermis. The cortex consists of four layers 
of oblong ovoid cells, increasing to six or eight layers with 
large intercellular spaces in older organs. The outer layers 
contain numbers of raphide-cells, and numbers of more or 
less anastomosed clumps of filaments. The internal layers 
contain great masses of closely interwoven hyphae, forming 
an irregularly ovoid or globose body lying against or nearly 
surrounding the nuclei. The general appearance of the 
