268 Scott and Brebner . — On Internal Phloem 
still, where the fusion with the external phloem is complete 
the cambium appears in its typical position entirely within 
the phloem. 
The figures i to 5 are selected from a series of 140 suc- 
cessive transverse sections. Fig. 1 shows the structure of the 
hypocotyl above the point where the internal phloem has be- 
gun to pass out, or at least before any of it has passed beyond 
the primary xylem. In Fig. 2 the pith is already much smaller, 
and three of the internal phloem-strands are shown on their out- 
ward course, having already passed the primary xylem-groups 1 . 
In Fig. 3 the pith is still smaller, much of the internal phloem 
has passed out, and one large group is seen imbedded in the 
secondary wood outside one of the cotyledonary traces. Fig. 4 
is from a region, where the minute pith is almost entirely 
occupied by such internal phloem, as has not yet made its 
exit. The xylem of each pair of cotyledonary bundles is here 
united into a single group, and beyond one of the two groups 
thus formed, a phloem-strand is shown, on its way to join its 
neighbours outside. Lastly, Fig. 5 shows the typical root- 
structure. The two cotyledonary xylem-groups here form 
a single diarch plate. There is no pith, and all the phloem 
has become external. 
It will be seen that the sections were taken from a plant, 
which, though young, already showed some secondary thick- 
ening. We have stated above, that at certain points of the 
transitional region the cambium crosses the outgoing phloem- 
strand. The question arises : does the cambium at these 
points form lignified secondary wood, so as completely to 
interrupt the continuity of the internal and external phloem, 
or does it keep up this continuity by the interposition of 
secondary phloem-elements? We have no doubt that the 
latter is the truth. In the sections drawn, some of the ele- 
ments of the outgoing strand are most probably of secondary 
1 It would e just as correct to speak of the xylem passing inwards as of the 
internal phloem passing outwards, the question being simply one of relative 
position. The description given in the text is, however, preferable, for the point 
to be emphasized is the ultimate junction of the internal with the external 
phloem. 
