158 MR. P. GROOM—CONTRIBUTIONS TO THE 
from those walls which belong also to cells containing hyphe,. 
and these pegsare directed away from these latter cells. In fact, 
the pegs are much modified and thickened walls of arrested 
hyphe. The following facts show that this interpretation is 
correct :—(1) Often an axial canal can be seen running up the 
centre of the peg and ending blindly ; this canal is continuous 
with a pit traversiug the cell-wall. (2) In all cases which obser- 
vation can establish the occurrence of the pegs coincides with the 
distribution of mycorhizal hyphe in the region of cells with thick 
modified walls. Thus, in a root-apex frequently mycorhizal hyphe, 
inside and outside the root, can be seen only on one side of the 
root; the peculiar pegs occur only in connexion with the thick- 
walled superficial cells of the same side. In cells not lying on 
the surface the pegs occur only on walls of cells adjoining other 
cells which have mycorhizal hyphx. This latter statement holds 
not only for cells in exocortical regions, but for certain cortical 
cells of root and rhizome lying much further inwards, and having 
thick lignified and pitted walls. (3) In these last-mentioned 
cells transition-stages to hyphe may be seen. (4) Often the 
pegs are bluntly branched. (5) In aerial parts smaller but 
similar pegs occur in connexion with thick-walled epidermal cells ; 
and in connexion with sclerenchyma-cells where wounds have 
permitted the ingress of hyphe. (6) In the apex of the root the 
thick walls of the superficial cells turn yellow with caustic potash ; 
but the pegs remain white. Thus, the pegs are the ends of 
hyphe the growth of which has been arrested, whilst the wall 
has thickened. I suppose the arrested growth is occasioned by 
the rapid deposit by the cell of layers of cell-wall on to the hyphal 
wall. For I could not succeed in ever tracing the stratification 
of the pegs through the cell-wall. The stratification, on the 
other hand, appeared continuous with that of the cell-wall. At 
first it suggested itself that these pegs might belong to a fungus 
which was not symbiotic, but which would injure the roots. 
This view is negatived by the fact that deep in the cortex the 
mycorhizal hyphe can be traced directly into the pegs. In 
addition, the pegs occurred in all three plants which were 
examined. Outside the root, the hyphe originally continuous 
with the pegs die off; so the pegs cannot be regarded as anchoring 
organs or as functioning as haustoria (except possibly for a very 
short and evanescent period). 
