432 Arber . — On the Roots of Medullosa ang licet* 
in front of the point of origin of the xylem-elements. This 
protrusion becomes more and more marked, until finally the 
rootlet becomes cut off, and a new growth of periderm com- 
pletes the outer sheath of tissue of both the root and rootlet. 
For a short time after it has become free, the rootlet lies 
in a groove or inflection of the periderm of the root. One 
of these grooves can be seen in nearly every transverse section 
(PL XX, Figs, i and 6,gr.) y opposite a protoxylem-group. 
Conclusions. 
The examination of the roots of Medullosa in the Binney 
specimen has resulted in a more complete knowledge of 
the thin-walled tissues which lie between the xylem and the 
periderm. The most noteworthy points are, the presence of 
a thin zone of phelloderm, the structure of the phloem, and 
the discovery of lateral sieve-plates on the phloem-elements 
of both the stem and roots. In the phloem of Medullosa , 
we have another point of agreement between Medullosa and 
Heterangium. The structure of the root of Heterangium 
tiliaeoides is at present unknown, but the phloem in the roots 
of Medidlosa anglica closely resembles that of the stem in the 
former species. 
