642 Wallace.—On the Stem-Structure of 
phloem makes a late appearance and is never large in amount. 
The bundles of the seedling are unmistakably collateral 
(Figs. 7 and 8), as also are those of younger internodes of the 
older plant (Fig. 9). Sections of a stem over | inch in 
diameter and a long distance from the apex showed no traces 
of differentiated medullary phloem (Fig. 9). It is not until 
a considerable amount of secondary tissue has been produced 
within the bundles that the elements of the medullary phloem 
proper to each begin to be differentiated. The appearance 
of the first sieve-tubes is preceded by the formation of a 
greater or less amount of small-celled meristem on the inside 
of the wood of each bundle (Fig. 9). This small-celled 
meristem arises by division of those cells which abut inter¬ 
nally on the protoxylem. The primary sieve-tubes are dif¬ 
ferentiated at the inner limit of the meristem and form an 
irregular semicircle around the protoxylem: they are thus 
separated from the wood by one or more layers of meristem. 
The further increase in amount of the medullary phloem 
takes place by means of a medullary cambium formed in the 
following way:—Cells of the above-mentioned meristem, 
which lie on the outside of the first-formed sieve-tubes (i. e. 
nearer the wood), elongate in a direction which is radial with 
reference to a point occupying the centre of the protoxylem, 
and divide tangentially with reference to the same point. 
A somewhat fan-shaped arrangement of cells results (Fig. 10). 
The inner elements so formed pass over into phloem, but no 
centripetal wood is formed. 
Medullary phloem does not arise simultaneously in relation 
to all the ten bundles of an internode. The three large inner 
bundles first acquire it; then the larger of the remaining two 
inner bundles; next the corner bundles, i. e. those of the 
outer ring; and finally the remaining inner bundle, which, 
it will be remembered, was the last to develop its xylem 
(Figs. 2, 3, 4). As secondary thickening in the bundles pro¬ 
ceeds, the more superficial layers of external phloem tend to 
become crushed against the resistant sclerenchyma. The me¬ 
dullary phloem, on the other hand, experiences no compression, 
