5 



I 



On the Growth of Leaves. 275 



subject to the 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. 



ji The generally received opinion with regard to the growth of 

 ||feaves 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 

 with 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. Schleidenf 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 % 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 Algse 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- 

 ^pment of the outer marginal cells. The same occurs in the 



aracea. In the Hepaticce, if the leaf consists of a branched 

 iries of cells (as in /. tricophylla and /. 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 divide 

 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 



ocess, and so on till the whole growth in width is completed. 



the Hepaticce when the leaves are layers or plates of cellular 

 ssuc like those of the Mosses just described, the process is 

 similar, except that they appear generally to have several apical 



primary cells. When the leaves are more than one layer thick, 



is often the case in the midnervc, septa arc found in the cen- 



f* Elein. Philosophiae Botan. i. 438. 

 ft Grundz. der Wiss. Botanik, 2iid edit. vol. ii. p. 172. 

 I Schl. and Nagcli's Zeitschr. fiir Wiss. Bot. part 3, 153. 



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