FURTHER DEVELOPMENT OF WELWITSCHIA MIRABILIS, 581 
members of the central groups. Successive sections higher up 
the stem show a gradual deviation from this arrangement. ‘The 
four central groups split up into smaller central bundles 
(fig. vit, 5); these, however still retain a position obviously 
related to four centres. Above fig. vir, 4, it was not possible to 
identify the primary bundles. As they proceed again further up 
the stem, the bundles nearer the centre of the transverse section 
begin to rotate separately on their axes, so that each assumes a 
position with its xylem directed towards the centre and its 
phloem towards the periphery of the section (cf. the rotation of 
bundles in the hypocotyledonary stem of the seedling; former 
paper, p. 22). At the same time the courses of the bundles 
diverge, so that the whole bundle system becomes separated into 
(a) a group of bundles which arrange themselves into two wavy 
nearly parallel lines, and which are clearly the bundles which 
come in from the plumular leaves (blatt-spurschicht) ; this is 
seen to be the case in fig. vi1, 6, where the section includes the 
margins of the plumular leaves; (4) a central group of small 
bundles, which will be afterwards seen to run up into the lobes 
of the crown; and (c) a peripheral series, which are regularly 
arranged with their xylem towards the centre and their phloem 
towards the periphery. Between these various series are to be 
seen bundles which pursue a horizontal course, and keep up con- 
nection between them. 
For the further elucidation of the bundle system of the apex 
we must refer to longitudinal sections. Since the lobes of the 
crown have a bundle system of their own, it is to be expected 
that the arrangement of bundles will appear different according 
as sections are made in a median plane (fig. 111, B), in which case 
the central lobes would be only slightly touched, or in a tangen- 
tial plane, which would pass through the lobe as in fig. 111, a. 
Such sections are represented in fig. vir, a4, B. In both of these 
may be seen the two deep leaf grooves, with the bases of the 
plumular leaves still adherent. Between the two grooves 
rise the lobes of the crown. In the one case (a) one of these 
lobes is cut through longitudinally, in the other (8) it presents 
its central edge. The dark shaded peripheral portions of these 
sections represent the masses of brown corky tissue, which are 
specially large at the upper part of the stock, and are produced 
by the degradations of portions of tissue originally active. The 
process of degradation extends as the plant becomes older, and 
even vascular bundles are often to be found included in the effete 
mass, especially in older plants. A similar degradation is also 
to be found at the apex of the lobes of the crown. The limit of 
the degraded tissue at the upper part of the plant is not marked 
by any very definite cork layer specially developed ; it appears 
VOL, XXI.—NEW SER. QQ 
