DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 109 



in which it is impossible to distinguish the ducts. Future 

 observations may teach us how this is achieved. 



In all the species which possess compound leaves, we have seen 

 the leaflets undergo, at their points of articulation, the same course 

 of modifications as the main stem, or rachis itself. With some, 

 suberification begins with the leaflets, in others, with the common 

 petiole. The leaflets of Fraxinus Ornus present the same appear- 

 ance as their rachis, by depositing similar cork layers. 



A very general occurrence, but of which we do not know the 

 full significance, is the abundance of crystalline aggregations, in the 

 tissues of the leaf, at the time of its fall, and however abundant this 

 may be in the neighbourhood of the articulation, we have not 

 hitherto been able to ascertain what part this phenomenon bears in 

 the work of separation of the leaf from its support. 



VT.— Theory of the Fall of the Leaf 



As the foregoing observations show,* it is the growth of the 

 peridermal layers which constitute the predisposing cause of the 

 fall of the leaves. This corky substance, however it may be de- 

 posited, causes the death of the tissues which it shuts off, by de- 

 priving them of the flow of nutrient matter. In fact, " suberous 

 tissue has only a short existence like living tissue. Its cells, when 

 once they are completely formed fsuberife'es), only contain air ; and 

 in this manner stop the passage of liquids" (SchachtJ. Its part 

 (role), is therefore to be at once destroyer and preserver; for 

 being that tissue above all others fitted to cover the scar, it hinders 

 evaporation in the denuded parts, and the spread of lesion in the 

 parenchyma. On this basis it is easy to comprehend how the 

 epiderm of the branches is destroyed when the peridermal layers 

 are formed : also, that, that these layers, enlarging their extent, 

 and extending from the outside of the pulvinus towards the centre 

 of the articulation (interligne articulaire\ the passage of the 

 plant juices, from the stem towards the leaf, will become more and 

 more obstructed, the more this layer increases in thickness and 

 extent ; and it will be seen that the final result of this new for- 

 mation, is the isolation of the parenchyma of the leaf from that of 

 the pulvinus. Because, take what plant you like, woody, herba- 



* This refers chiefly to the remarks on individual species, which have been 

 omitted on account bf their length.— Tr. 



