138 VEGETABLE ORGANOGRAPHY. 



and they deserve our attention a little longer than other 

 climbing plants. 



Most stems, even those which are perfectly upright, 

 present a spiral tendency in their development. Thus : 



1st. Trees but slightly branched, as the Fir, are fre- 

 quently seen to present this direction of the fibres in a 

 very decided manner ; and it is easily seen in the woody 

 body when it has remained for some time denuded of the 

 bark and exposed much to the air, which, by drying the 

 surface, causes spiral fissures. 



2d. Du Petit-Thouars has remarked, that the epider- 

 mis of trees with a smooth stem, as the Cherry and 

 Hydrangea arhorescens, is more easily taken off in a 

 spiral direction than in any other. 



3d. The primitive disposition of the leaves of Endo- 

 gens is spiral, and a great number of those of Exogens 

 are disposed in the same way, either naturally or 

 accidentally. In PI. 9, I have represented a very 

 remarkable monstrosity of the Mint, as an example 

 of spiral development in one of the families where we 

 should the least suspect it. Vaucher has made 

 known a curious example of Equisetuvi Jluviatile, the 

 stem of which presented the fibres twisted in a regular 

 spire. 



Is this spiral tendency of the fibres organic or physio- 

 logical ? Is the fact itself general ? I do not venture 

 to affirm this ; I am bound to mention these observations 

 as connected, it seems to me, with the history of twining 

 stems.'* Some of these, such as the Cohcea, present, in 



* This connexion with twining stems is also confirmed by a curious ob- 

 servation of M. Leopold de Buch — viz. that in several species, the direction 

 of the spiral twist of upright trunks appears as constant as that of twining 

 stems : thus, lor example, according to tliis observer, the Horse-chestnut 

 and Common-chestnut turn in contrary directions. 



