282 Scott and Brebner. — On Internal Phloem 
detailed description. In Oenothera longiflora, Jacq. we were 
able to confirm Weisses observations as to the presence of 
centrifugally formed interxylary phloem in the root of this 
genus 1 . A large amount of secondary phloem-parenchyma 
is formed here, but the groups of sieve-tubes are small on 
both sides of the cambium. 
In the following bicollateral plants the roots investigated 
showed external phloem only : — 
Myrtus mucronatus , Cambess. Ly thrum Graefferi , Ten. 
Calystegia septum?, R. Br. Apocynum cannabinum , L. 
W illughbeia fiavescens , Dyer. Willughbeia firma, Bl. 
Crypto stegiagrandijior a, R. Br. Dischidia nummularia , R. Br. 
Hoy a pendula, Wight et Arn. Periploca graeca, L. 
Daphne Mezereum , L. 
In all these cases it is probable that the transition from 
stem to root takes place as in Ipomoea and Browallia , and 
that the medullary phloem of the stem is continuous with the 
external phloem of the root. 
Our conclusions as regards the roots may be summed up as 
follows :■ — 
1. The majority of the plants examined with bicollateral 
bundles in the stem showed normal structure of the root, the 
medullary phloem (to judge from the cases specially investi- 
gated) turning outwards in the hypocotyl with the protoxylem- 
groups. It is thus continuous with the external phloem of 
the root-system. 
2. A certain number of roots among the plants in question 
have interxylary strands of phloem. These may be (a) 
primary , (b) secondary, or (c) tertiary. The first, as regards 
the main root, form the direct downward prolongation of the 
medullary phloem of the stem, the phloem-strands running 
down through the wood, instead of turning outwards to join 
the external phloem. The secondary interxylary phloem, 
which may be limited to the root or extend also into the 
stem, is in the plants here described formed centrifugally, on 
1 Loc. cit., Flora, 1880. 
2 Cf. however the paper on Ipomoea versicolor L Scott, Ann, of Bot No. XVIII. 
