ES pea THE OAK 
distinct outlines; these alternate groups are known as 
phloém, and we may shortly examine the elements of 
which they are composed, as before, by comparing 
sections of various kinds. 
Here, again, we find the chief structures inthe phloém 
are also vessels—t.e. lone’, tubular organs—but very dif- 
ferent in detail from the vessels of the xylem. 
In the first place, their walls are thin and soft, and 
composed of the unaltered cellulose which is so charac- 
teristic of young cells (instead of being hard like the 
lignified walls of the xylem vessels); then, again, they 
contain protoplasm and other organised cell contents, 
instead of merely air and water. Finally, they are not 
so completely tubular as the typical xylem vessels are, 
because the transverse septa of the constituent cells are 
not absorbed, but are merely pierced by fine strands of 
protoplasm, and therefore look like sieves when viewed 
from above—whence the name ‘ sieve-tubes.’ In the 
phloém also we find cells—phloém-cells—packed in 
between the sieve-tubes. 
If we shortly summarise the above we find that the 
root consists of an axis-cylinder surrounded by a cortex 
and the piliferous layer. At the tip the whole is 
covered by the root-cap, which is organically connected 
with the embryonic tissue which forms all these struc- 
tures. The axis-cylinder is somewhat complex; it is 
sheathed by the endodermis and the pericycle, the 
latter of which gives origin to the new rootlets. Inside 
the pericycle are the vascular bundles running up and 
