382 LECTURE XVI. 



the etiolated leaf of a seedling varied from 1375 2571, and 

 in the normal leaf from 1429 2273. These figures clearly 

 prove that cell-multiplication by division takes place in leaves 

 in darkness. The arrest of growth is not due to a diminished 

 formation of cells in the leaf, but to an interference with 

 the growth of the cells formed. The arrest of growth may 

 have, however, the effect of diminishing the activity of cell- 

 division, for the division of cells is dependent upon their 

 growth. 



Other observers, such as Rzentkowsky, Mer, and C. Kraus, 

 have correlated the smallness of leaves in darkness with the 

 excessive elongation of the internodes of shoots, and regard 

 the latter as the cause of the former. In support of this view 

 those cases may be adduced in which, as in the Beet, the 

 leaves are fairly well-developed in darkness whilst the stem 

 exhibits no excessive elongation, or those, such as Allium, 

 Iris, and other Monocotyledons, in which the leaves become 

 excessively elongated in darkness whilst the stem does not. 

 But the converse of this is not true, namely, that excessively 

 elongated stems always bear very small leaves. For instance, 

 according to Rauwenhoff, the shoot of Fritillaria imperialis 

 becomes excessively elongated in darkness, whereas there is 

 no corresponding difference in size between the leaves of an 

 etiolated and of a normal plant. Again, it has been shewn 

 by Senebier, G. Kraus, and Godlewski, that if a leaf-blade be 

 kept in darkness whilst the rest of the plant is exposed to 

 light the blade remains small, though it is true, as Sachs' 

 observations prove, that the leaf-blade may attain a greater 

 size under these conditions than it does when the whole plant 

 is kept in darkness. It may be indeed admitted that the ex- 

 cessive consumption of plastic material in the rapid growth of 

 the stem, tends, when the store is limited, to affect prejudicially 

 the growth of the leaves. Godlewski has in fact found, in the 

 case of seedlings of the Radish, that the excessive elongation 

 of the hypocotyl tends to diminish the development of the 

 cotyledons. But, taking all the facts into consideration, the 

 excessive elongation of the internodes of etiolated plants 

 cannot be accepted as the cause of the smallness of the leaves, 



