Arber.—On the Roots of Medullosa anglica . 427 
zone is exceptionally well preserved, and the evidence as 
to the nature of the tissues composing it is probably more 
complete than in the specimens which have previously been 
described. These tissues will be referred to presently in 
greater detail. 
The rest of the root consists of an exceedingly regular 
triarch strand of xylem. The protoxylem-elements (p.x.) 
lie on the inner margins of three large bays or concavities, 
which occur at regular intervals in the almost circular outline 
of the strand. The elements of the primary wood can be 
seen extending from the protoxylem-groups to the centre of 
the root. 
The secondary wood (x 2 .) is formed of three large plates 
of radially disposed elements, with convex outer surfaces. 
The rays of woody elements composing the plates are 
generally one or two rows of tracheides, with multiseriate 
bordered pits on their radial walls. The medullary rays are 
often perfectly preserved. The parenchymatous cells of the 
ray are rounded, except where they have become elongated 
radially, probably as the result of stretching accompanying 
the increase in the dimensions of the secondary wood. 
As a rule no secondary xylem-elements are formed opposite 
the protoxylem-groups. The vascular system of the secondary 
roots has, however, its origin in this position, and the xylem- 
strand, seen in one of the concavities ( r.x .), is in connexion 
with a rootlet, which arises at some distance further along the 
root. 
These roots differ somewhat in shape from some of those 
previously figured. The structure is, however, identical in 
both cases. Dr. Scott has made out in considerable detail 
the minute anatomy of the triarch strand, and I have nothing 
to add to our present very complete knowledge on this 
subject. 
We may now consider in greater detail the structure of 
the zone of thin- walled tissue lying internal to the periderm, 
abutting on the convexities of the secondary xylem, and 
usually filling the concavities opposite the protoxylem-groups. 
F f 2 
