in the Root and Stem of Dicotyledons . 271 
gradual one, and is not completed until the group has been 
left far behind by the main cambium (Fig. 6). 
The elements of the phloem of the root resemble those of 
the stem, but the joints of the sieve-tubes are often very short. 
The transverse walls both of the external and of the inter- 
xylary sieve-tubes may be horizontal, bearing a single sieve- 
plate, or inclined, in which case they show a series of plates. 
In size the interxylary are quite equal to the external sieve- 
tubes, both often exceeding “03 mm. in diameter. 
Roots 1 cm. thick show essentially the same structure. By 
this time a pericyclic periderm has been formed. The xylem 
is more parenchymatous than ever. Not only has an immense 
amount of fresh parenchyma been added by the cambium, but 
the older parts have undergone dilatation, which starts from 
several centres and leads to the obliteration of some of the 
elements. This process is accompanied by the formation of 
new strands of phloem by the subdivision of cells in the 
dilated xylem-parenchyma. 
The interxylary phloem-groups of the root then may arise 
in three different ways : — 
1. By the division of cells of the primary conjunctive par- 
enchyma. 
2. By direct centrifugal development from the cambium. 
3. By late division of cells of the older secondary xylem- 
parenchyma long after its origin from the cambium. 
We might for convenience distinguish the three varieties as 
primary, secondary, and tertiary phloem-islands. From what 
has already been said, it will be clear that the distinction 
between them is not always quite a sharp one. 
In the thicker roots horizontal or oblique anastomoses ot 
the interxylary phloem-strands are often found. 
The transition from stem to root could only be examined in 
an old plant, as no seedlings were available. For one or two 
internodes above the hypocotyl the wood of the stem contains 
minute phloem-islands. They extend up as far as the wood 
remains parenchymatous, generally occurring near the groups 
of tracheae. Careful study of successive sections led to the 
