FURTHER DEVELOPMENT OF WELWITSCHIA MIRABILIS, 577 
The lateral roots appear externally to the protoxylem masses, 
and are produced by the activity of the pericambium only, the 
cells of the bundle sheath apparently taking no part in the 
process. 
Transition from Root to Hypocotyledonary Stem. 
We have found no difficulty in recognising the relation of the 
structure of the older root to that of the young root, as described 
in my former paper (cf. l.c., fig. 22). As we now proceed to 
trace the transition from true root to hypocotyledonary stem, we 
must bear in mind the structure of the corresponding part of the 
young seedling as there described. It has already been shown 
how in the lower portions of the main root the two originally 
separate protoxylem masses unite centrally to form a single xylem 
plate, but that for some distance below the point of transition 
from root to stem this coalescence does not take place, the centre 
of the root being occupied by a mass of parenchyma. Here, as 
in other parts of the plant, the parenchyma retains its activity, 
and is capable of increase. This may be observed in plants at a 
comparatively early stage. In those sent me by Chev. Monteiro 
(four months old), divisions in this medullary tissue were already 
observable. 
The course of the vascular bundles at the point of transition 
from root to stem was traced carefully in the plant represented in 
fig. m1, and the following description is based chiefly upon results 
obtained from it :—The main root retains throughout its length 
the same arrangement of its vascular system (fig. 1v), the only 
change being that higher up (7.e. nearer the stem) ; the peripheral 
system is more complicated. In the series of figures (v1, 1—6) 
which illustrate the transition from root to stem, the younger 
peripheral bundles, as well as the spicular cells and sclerenchyma 
fibres, are omitted. In all cases the xylem is shaded, while the 
phloem is left white. The numbered dotted lines in fig. 11 
show the points at which the successive members of the series 
fig. vi were cut. 
If we compare fig. 1v with fig. vi, i, we see at once a difference 
in the arrangement of the central vascular group. In the latter 
figure it is divided into two parts by a plate of parenchymatous 
tissue. The protoxylem masses can easily be distinguished (pr. 
ry.). They have the same relative position as in fig. iv. Pro- 
bably the fig. 22 of my former paper represents a section cut at 
a corresponding point in a younger plant: since a central fusion 
of the vascular tissues did not take place at this point in the 
young root, the secondary products, as seen in the older roots, 
also remain permanently separated by a plate of parenchymatous 
