276 Mr. A. Henfrey on the Progress of Physiological Botany : 
tral cells parallel with the surface of the leaf, and the process ex- 
tends outward from the central to the lateral cells according to 
the specific peculiarity of the plant, but it is always the central 
cell which first divides. 
In the Lycopodiacece and Equisetacece the leaves also grow in 
length and breadth by the development of their apical and mar- 
ginal cells. The Phanerogamia follow the same law. In an im- 
perfect leaf, the cells at the border and apex are full of the 
homogeneous mucilage (protoplasm), while in the others it is 
already transformed into yellowish or greenish granular matter. 
Sometimes the formation of the septa may be observed in the 
marginal cells. In thin leaves the increase in length by the di- 
vision of the apical cell may frequently be observed during the 
growth of the plant. 
The fact that the leaves of Phanerogamia grow at the apex and 
borders and not by the base, is most easily seen in compound or 
much-divided leaves. As a general rule the lateral axes shoot out 
from the main axis in succession from below upwards ; in like 
manner grow the tertiary axes (when present) from the secondary. 
In Astragalus (which is figured by the author) it is shown 
that the uppermost leaflets are the youngest, the lowest the old- 
est and largest. 
In Utricularia the growth of the leaf originally and of the di- 
visions subsequently, may be seen to occur by continual deve- 
lopment at the apices of the main axis and the divisions. In 
Myriophjllum however was found an exception, the upper lobes 
of the leaf being formed first. 
The thickness of the leaf, the various inferior layers of the 
epidermis and the parenchyma depend on another mode of 
growth, which Nageli calls, in opposition to peripheric al cell- 
developmentf cell-development in every direction [allseitige Wachs- 
thum) . Three forms of this occur : I . It is either absent or 
merely follows at a little distance the development of the apical 
cell from below upward and soon ceases. In this form the de- 
velopment of the cells often ceases in the lower part of the leaf 
before it is complete at the borders. Utricularia is an example. 
2. The development in every direction occurs simultaneously in 
all parts of the leaf, which completes its peripherical growth very 
rapidly. The growth in this form usually ceases in all parts 
about the same time. 6. The development in every direction 
begins, after the rapid completion of the peripherical growth, 
either only or at all events principally on the upper part of the 
leaf and extends downward. It ceases at the base last. 
There is also sometimes an abnormal growth in every direction, 
which occurs either in particular cells or in the whole tissue, and 
does not appear to be subject to any laws. 
