43i 
Phloem in the Stems of Dicotyledons. II. 
the branch-system may arise both in this way and also de novo in its pith- 
tissue. The small peripheral phloem-strands persisting in the higher part 
of the stem and peduncle become part of the ordinary vascular system of 
the flowers. 
The above structure is also exhibited by L. Scariola , L., L. saligna ) L., 
and L. sativa , L. 
In L. ( Mulgedium ) alpina , Bth. and HK., L. Plumieri , Gren. and Godr., 
L . Bourgaei , L. macrantha , C. B. CL, and L . perennis , L., the medullary 
Fig. 5. Lactuca virosa. Segment of vas- Fig. 6. Z. virosa. Ditto from upper part 
cular ring from lower part of stem ; in the pith of stem, showing are-shaped internal-phloem 
are scattered bundles and phloem-strands, x 4. strands, x 4. 
strands (both bundles or of phloem only) are few in number or completely 
undifferentiated and vestigial. In some species they only occur at a certain 
level of the stem, in others solely at the nodes. 
In L . hastata , DC., and L . macrophylla , A. Gray, medullary strands are 
completely absent. 
Leaf. 
In L. alpina and L. Plumieri there is a complete vascular ring. 
A medullary system of very small scattered bundles and phloem-strands 
occurs close round the central lacuna, and also a peripheral system of fewer 
Fig. 7. Lactuca Plumieri. Petiole, show- 
ing vascular ring ( vr ) and medullary strands 
(ms ) ; la, lacuna, x 4. 
Fig. 8. Lactuca alpina. Petiole, showing 
vascular ring and medullary strands (ms ) ; 
la , lacuna, x 4. 
strands (Figs. 7 and 8). The medullary system ends blindly in the tissue 
below, as do also the smallest bundles of the ring. This structure is also 
shown by one or two other species. 
