278 Mr. A. Heiifrey on the Progress of Physiological Botany. 
alone ; and although he is in accordance with most authors in 
stating that the fronds of Algse grow by their apices and 
borders, there are also exceptions here — for instance in Lami- 
naria digitata, where the new frond is produced by the expansion 
and development of the stalk-like part of the old one. Passing 
by these, it is evident that the leaves of Mosses and Hepaticce 
differ widely from those of the Phanerogamia in general in their 
development, though they bear considerable resemblance to such 
as those of Utricularia, &c., and many of the Monocotyledons. 
The leaf of a Dicotyledon originates as a little papilla of cel- 
lular tissue : if it is a lobed leaf, these lobes appear in succession ; 
thus at first we see a little cone, then a three-lobed fiattened pa- 
pilla, next a five-lobed, and so on ; and here it is difficult to say 
how we shall prove, how in the five-lobed form the intermediate 
lobes originated — whether they are new ones, or the two original 
lateral lobes pushed up by two succeeding lobes — since we can 
only make observations on separate leaves, not see them grow ; 
but as it is clear that the papilla does grow at the base, becoming 
narrowed into a petiole and pushing the whole of the blade up, 
we have a right to assume that the leaf does in the first instance 
develope at its base. But then we must not generalize for the 
whole growth from this, since as soon as the petiole is distinctly 
formed, the petiole and the lamina have distinct growth ; and now 
the leaf in its expansion by the multiplication of its cells must 
grow chiefiy at its borders, since the centre of the base, that is, 
the point of junction with the petiole, must retain its relative 
position, and may therefore be considered as the point of de- 
parture of all growth in the lamina ; so that as the apex and the 
borders are subsequently at a greater distance than at first, they 
must develope away from it in all directions, whether by mere 
marginal and apical alone or by central development also, since 
in the latter case the border must grow to make room for the 
growth in the centre. Nageli says that the growth by expansion 
of the individual cells commences at the apex, but it would very 
often be difficult to distinguish whether this expansion at the 
apex depends on development of cells or actual expansion of those 
already formed ; he probably reasoned from analogy here in re- 
gard to the Phanerogamia. Most experiments have shown the 
expansion to be tolerably simultaneous throughout. 
The leaves of Monocotyledons, such as those of the common 
bulbous plants at least, appear to develope chiefiy, if not solely, at 
the base. In those which have petioles there must be a differ- 
ence, but in such we observe the growth or actual development 
to continue longer in the petiole than in the blade. 
The forms of leaves differing so much even in the same spe- 
cies, often in consequence of difference in the amount of paren- 
