275 
On the Growth of Leaves. 
subject to tbe same law of growth^ but they present more ano- 
malies than any other parts. He states that these are however 
more apparent than real, and promises to give a detailed account 
at some future time. 
The generally received opinion with regard to the groudh of 
leaves is that, in contrast to the stem, they grow at their base 
only, and their summits are therefore considered to be the oldest 
parts. 
Link* however states that the leaf appears at once in the bud 
udth all its parts formed, and that it then grows by interstitial 
development, but mentions one exception, in the Walnut, where 
the leaves appear ternate or tripartite at first, and the other lobes 
appear subsequently. Schleidenfi declares that the apex is the 
oldest part and the base the youngest, and that although the 
process of development within the leaf may increase its size and 
influence its internal structure, it has no power of determining 
its form ; while to complicate the subject still more, Nageli J has 
just published a paper advocating the diametrically opposite opi- 
nion. His views are so definitely expressed that they well merit 
an examination. 
In the first place he draws a marked distinction between two 
modes of growth which necessarily exist in all leaves, viz. 
1. growth by cell-formation, and 2. growth by the expansion of 
the cells. Considering the fronds of the Algae to represent leaves, 
he first points out how these grow by their apices and borders, 
the increase in length resulting from the continual division of 
the apical cell {scheitel-zelle), and the increase in breadth, where 
the lobe or branch consists of several parallel rows, by the deve- 
lopment of the outer marginal cells. The same occurs in the 
Characece. In the Hepaticce, if the leaf consists of a branched 
series of cells (as in J. tricophylla and J. setacea), it grows by the 
apical cells as in the Floridece. If the leaf is a layer of cells — in 
the Mosses it possesses one continually developing apical cell and 
the lateral growth is simultaneously efiected by the division of 
the cells left behind as it were by the apical cell, which dhdde 
by a septum at right angles to that of the primary cell, and the 
first two producing four, the outer one of each pair repeats the 
process, and so on till the whole growth in width is completed. 
In the Hepaticce when the leaves are layers or plates of cellular 
tissue like those of the Mosses just described, the process is 
similar, except that they appear generally to have several apical 
or primary cells. MTien the leaves are more than one layer thick, 
as is often the case in the midnerve, septa are found in the cen- 
* Elera. Philosophise Botan. i. 438. 
t Grimdz. der Wiss. Botanik, 2nd edit. vol. ii. p, 172. 
X Schl. and Nageli’s Zeitschr. fiir Wiss. Bot. part 3, 153, 
