FURTHER DEVELOPMENT OF WELWITSCHIA MIRABILIS. 579 
increase were due to a peripheral cambium layer, but to an 
intercalary growth throughout the section. 
From the figures of Hooker illustrating the structure of older 
plants, and from observations of dissections of older specimens 
with the naked eye, I conclude that after a certain time this 
intercalary activity is diminished at the centre of the plant, and 
becomes localised in the peripheral portions. 
Up to fig. v1, 4, we have been able to trace the relationship of 
the primary bundles of the seedling to the vascular system of 
the older plant. We must, however, leave the study of the 
upward course of the bundles at this point; before we are able 
properly to estimate the relations of the vascular tissues of the 
upper part of the stem, we must become better acquainted with 
the external conformation of the apex of the plant. 
Apex of the Plant.—Eaternal Characters. 
From plants of three to four months growth I was able to 
obtain early stages in the development of those two structures, 
which he between the cotyledons; these were mentioned in my 
former paper (p. 19, note), but I was not then able to make any 
definite statement concerning them. . 
If a thick horizontal section be cut through the apex of a 
plant of about four months growth, it will be found to include 
(1) the bases of the cotyledons, the two being distinguished only 
by two indentations of the margin of the section; (2) the bases 
of the plumular leaves (yp. /. fig. vit) ; (3) the apical cone of the 
stem (ap.), which appears hardly any further advanced than in the 
younger seedlings described in my former paper (fig. 15 a); and 
(4) the two structures in question, which lie between the plumu- 
lar leaves on either side of the apical cone (/.c., fig. vit). In this 
figure they appear of unequal size; this appearance is, however, 
due to the fact that the plane of section was not exactly horizontal 
in the case figured. In reality they are as a rule nearly equal in 
size. (Note.—Fig. 10 of former paper is an exception to this.) 
I have observed no sign of fresh lateral appendages either on 
them or on the apical cone, with the exception of the fertile 
branches, the origin of which is clearly adventitious, and will be 
dealt with later. As the plant increases in size these lateral 
cones grow more strongly than the apical cone, and overtop it, 
so that in the older plants they are the only prominent struc- 
tures to be found between the plumular leaves (fig. 111, and fig. 
10 of former paper). These two cones widen as the plant 
increases in age and form the crown of the plant. In plants 
with a crown of three inches in diameter it is possible still to 
trace a division of the crown into two areas corresponding in 
