584 F. 0. BOWER, 
the greater part of their tissues have been or are concerned in a 
similar division. Of course the rows of cells are not so clearly 
marked in the older parts as at the base of the groove, where the 
increase is most active: still they are easily traceable. I have 
accordingly tried to represent, by means of lines in fig. vil, A, 
the direction of these rows of parenchymatous cells (cf. Sachs, 
‘Anordnung der Zellen in jiingsten Pflanzen-theilen, fig. 5). 
Young spicular cells may often be found pushing their growing 
ends between the young tissues (for their development, cf. infra). 
Fresh vascular bundles also appear below the surface of the lips 
of the groove, but their course is, as a rule, so tortuous that it — 
can only be followed with difficulty. In the leaf, however, their 
course being rectilinear, it can easily be traced (fig. x). 
Cell division continues throughout the life of the plant in the 
directions described, relatively to the leaf groove. The fresh 
cells thus produced assume an approximately cubical form. 
Bearing these facts in mind, we shall be able to understand how 
- the further development of the plant proceeds. Since the divi- 
sions in the tissue immediately below the epidermis of both lips 
of the groove are mostly parallel to the surface of the groove, an 
increase in bulk takes place, both in the crown and in the upper 
part of the stock, mainly in a direction perpendicular to the sur- 
face, z.e. both the crown and the upper part of the stock become 
wider, at the same time the depth of the leaf groove does not 
materially increase. We have seen that below the base of the 
leaf groove the divisions are mainly in a direction perpendicular 
to the axis of the plant; hence there results an increase in 
length of the whole plant. But as the width of the crown 
increases the activity of the central portion falls off, till finally 
growth in length ceases in that part. The centre is, therefore, 
overtopped by the more actively growing peripheral part. Hence 
originates the cup-like form of the apex of old plants (ef. 
Hooker’s figures). We have already an indication of this in 
fig. x1. Finally, the divisions in the base of the leaf itself are 
mainly transverse, and thus the growth of the leaf is strongest 
longitudinally. For the verification of these several conclusions the 
figs. x and x11 of this paper should be compared with the figures 
of older plants given in Sir J. Hooker’s memoir. It will then 
be easily understood how, by the increase in width of the lobes 
of the crown, the bundles of the leaf trace (vascular strata of Sir 
J. Hooker) of the plumular leaves, which are in the young plant 
parallel to one another (fig. vir), become wedged apart. ‘As the 
plant grows the angle between them constantly increases till, in 
very old plants, the bases of the leaves almost lie in the same 
lane. 
; The section fig. x1 serves as an intermediate stage between the 
