600 
MR. F. O. BOWER OR THE COMPARATIVE MORPHOLOGY OF THE 
organ, without any midrib, and with a venation similar to that already known for the 
plumular leaf (compare De Bary, ‘ Yergl. Anat.,’ fig. 145). 
The plumular leaves are first formed after germination begins, as a pair decussating 
with the two cotyledons. The first stages of their development have not been traced, 
but in seedlings about six weeks old, in w T hich the plumular leaves are still very 
small, it may be clearly seen that their mode of growth already proceeds according to 
the same system as is maintained throughout the life of the plant. Longitudinal 
sections of such a seedling show that the tissues at the apices of the young leaves 
have already lost their meristematic activity, while spicular cells are already to be 
seen embedded in the parenchyma (Plate 40, fig. 52). Passing downwards from the 
obtuse apex of the plumular leaf towards its base, it is seen that the tissues become 
constantly more active, while at the extreme base divisions by walls perpendicular to 
the oi’ganic axis of the leaf follow in quick succession, and show that the leaf, while 
still small, owes its increase in length to the activity of a well-marked basal inter¬ 
calary zone. In such young leaves cut in median longitudinal section (Plate 40, fig. 
53) the tissues are seen to be arranged in regular layers, about ten in number, 
including the epidermis : no periclinal divisions appear in these layers, as a rule, 
during their development into mature tissue, so that they may be distinctly followed 
up into the more mature parts of the leaf. As the plant grows older there is an 
increase in the number of active layers in the intercalary zone, and therefore also in 
the mature portions of the older leaf. Thus in sections from an old plant it was 
found that the total number of layers was about 26. 
This increase in thickness is quite eclipsed by the increase in width of the whole 
leaf. As seen in fig. 52, the base of the leaf where it is inserted on the axis is the 
widest p>art of the whole leaf; in older plants the width of the mature portion of the 
leaf exceeds that of its insertion to a slight, but not very marked, degree. Thus there 
is but slight growth in the direction of the surface of the leaf as the tissues become 
successively developed. This being the case, sections through the base of the leaf 
will give a true indication of the distribution of the growth which brings about the 
increase of width of the leaf as the plant grows older. A series of such sections is 
represented in Plate 40, fig. 54, a-d: (a) shows the young plumular leaf about the 
same age as in figs. 52 and 53, with the two first vascular bundles already 
developed (i, i); in (b) a second pair of bundles is to be seen (ii, ii) between the 
margin and the first pair ; in ( c ) a third pair (iii, iii) has appeared in a similar position ; 
in (d) the three pairs are still to be seen, but between them other bundles have now 
been intercalated. 
On comparing these sections closely it will be seen that there has been a constant 
increase in distance between the bundles of the first pair. The same is the case with 
other parts of these sections, and a comparison of sections shows a continued but not 
rapid division of cells perpendicular to the surface. This transverse growth is not 
