Arber . — On the Roots of Medullosa ang licet. 429 
rather recalls that of the crystal-containing sacs of certain 
recent plants, e. g. Solatium tuberosum . There is, however, in 
this case no evidence that these ‘secretory sacs’ were of a 
similar nature. In a few cases the cell-contents were found 
to be much less altered than is usually the case, but even 
these afforded no clue to the nature of the original substance. 
The parenchymatous tissue in the upper portion of the 
photograph, occupying the concavity on the inner side of 
which lies the protoxylem-group (p-x.), is the very broad main 
medullary ray ( m.r .) 1 . Secondary xylem is usually absent in 
this position. The small xylem-strand (r.x.) seen in the photo- ‘ 
graph belongs to the xylem of a secondary root, which, as 
already mentioned, has its origin in this position. 
The Phloem. 
The photograph of a transverse section of a root, figured on 
PI. XX, Fig. 3, shows a portion of the thin-walled zone of 
tissue opposite one of the xylem convexities. The tissues 
seen here include the phloem, which was of course not present 
in the section just described. At the lower end of the photo- 
graph, a few of the elements of the periderm ( p.d .) and 
phelloderm are seen. Next comes a broad band of large- 
celled, irregularly disposed elements, the pericycle (p.c.). The 
‘ secretory sacs ’ with dark contents form large groups in 
this section. 
The upper half of the photograph shows a well-marked 
tissue, the elements of which are somewhat tangentially 
elongated. This is the phloem-zone (b.z.). The secondary 
phloem (< h 2 .) consists of radial groups of rather small cells, 
the sieve-tubes (s.t.) } alternating with rays of much larger 
phloem-parenchyma (bp.). The secondary bast as a whole 
has much the appearance of that of the stem of Heterangium 
tiliaeoides , Will. 2 . The groups of secondary phloem, corre- 
sponding to the groups of woody elements in the xylem- 
1 De Bary, loc. cit., p. 474. 
3 Williamson and Scott, Part III, Phil. Trans. Roy. Soc., Ser. B, vol, clxxxvi, 
1896, p. 761, PI. XXIX, Fig. 35. 
