6 



A similar section of the theca of H. populeum exhibits no annukis 

 whatever: hence the greater persistency of the operculum is easily 

 accounted for, there being no sj^ecial organ in this case designed for 

 its removal, or one which is so small and imperfectly developed as to 

 elude detection ; yet even here there is a sutural line visible, and 

 it may be supposed that it is through some inequality in the contrac- 

 tion of the parts adjacent, that dehiscence ultimately occurs. 



It is easy for an imaginative mind to form theories on any subject, 

 and to support them by plausible arguments, but it seems legitimate 

 to apply the preceding facts in explanation of the fall of leaves. In 

 these the petiole is usually* attached to the stem of the plant by an 

 articulation, where probably the cellular tissue will be found to be 

 more lax than the adjacent parts. The woody tissue of the petiole is 

 obviously less contractile than the parenchymatous tissue ; and as a 

 period must arrive when the cells acquire their maximum of indura- 

 tion, and cease to act in discharge of their original functions, from 

 what may be called a superinduced ossification of parts, previously 

 soft and elastic, the moisture of the sap being no longer supplied to 

 preserve their pristine bulk, the softer cells contract in drying, sepa- 

 rate from the adjoining parts, and the woody tissue being also ren- 

 dered fragile by drying, the whole weight of the leaf is made to fall 

 upon those parts which have not contracted, and are too weak of them- 

 selves to sustain the burden. 



If this explanation is founded in Nature, it will be found that leaves 

 remain longer upon the trees in seasons of continued moist weather, 

 and for a shorter period when the autumn is a dry one. The return 

 of wet weather after drought may hasten the fall of the leaf, by 

 adding mechanically to its weight, and so also may wind, by pro- 

 ducing lateral fracture of the woody parts of the petiole. The ques- 

 tion must be considered apart from those circumstances. 



It is to be hoped that this subject will be discussed by other 

 observers ; and if these views shall prove to be erroneous, 1 shall still 

 be happy to have supplied a stimulus to some one who shall for- 

 tunately arrive at more accurate conclusions. 



W. Wilson. 

 Orford Mount, near Warrington, 

 December 6, 1844. 



* In the holly there is no articnlation, and the leaves seem to be pushed off at the 

 end of the second year, by the swelling in spring of the bvauch immediately above 

 them, rather than to fall off of themselves -. a similar event occurs in Viscum album, 

 at the end of the first year. 



