572 ¥, 0. BOWER. 
covered with a thick fluffy layer of tissue (effete corky tissue) of 
a light yellow colour. This gives place in still older plants to a 
darker and even black covering of greater strength and hardness. 
These remarks apply also to the lower part of the hypocotyle- 
donary stem. ‘The fluffy layer, being very friable, could not 
easily be cut into sections by the razor; it will therefore not be 
represented in the figures of sections accompanying this paper. 
Internal Structure.—Root. 
It was shown in my former paper (p. 25) that the cortical 
tissue of the root, being cut off from physiological connection 
with the central cylinder, ceases to grow, and is thrown off, so 
that the older root consists of tissues derived only from the cen- 
tral cylinder. We will now trace the further development of 
that cylinder. Its periphery (as seen in fig. 22, 1.¢.) is oceupied 
by a formative tissue derived from the pericambium. This gives 
rise to a constantly and successively renewed cork tissue, which 
is always to be found covering the root externally. Beyond its 
presence it is not of any special interest; we may, therefore, 
pass on to the consideration of the internal tissues. 
In the specimens described in my former paper the fusion of 
the diarch xylem system at the centre of the root was not 
observed (1. c., p. 25, fig. 22). The two xylem masses were in 
all cases separated by a parenchymatous tissue. It is true Ber- 
trand (‘ Ann. d. Sci. Nat.,’ série v, tom. xx, p.9) has described 
the central fusion of the xylem masses of the lateral root- of 
Welwitschia, but the question as to the main root still remained 
open. On comparison of older plants, however, it appears that 
in the main root, for a short distance below the transition from 
stem to true root, such a fusion does not occur, the centre of the 
transverse section being permanently occupied by a mass of 
parenchyma. As this tissue bears an important relation to the 
further development of the part where it occurs, it will be 
again referred to later (cf. fig. 22 of first paper, which was 
taken at a point close below the feeder). In the lower portion 
of the root a complete coalescence of the two originally separate 
xylem masses takes place, so that a single plate is formed; the 
junction is followed by a further development of xylem at the 
central part of each side of the plate, produced by an active 
cambium layer, which. lies between the primary phloem and the 
xylem plate. The secondary xylem thus produced appears as a 
narrow band on each side of the xylem plate, the two bands being 
separated laterally from one another by very broad parenchyma- 
tous rays. ‘The whole xylem system thus produced attains a 
stellate form (fig. 1.). Thus far the structure of the root of Wel- 
