240 Dr Daubeny on (he irritiiuis and 



form, flows down from the summit of every exogenous tree to 

 its base, and that the new layers of bark and wood produced 

 each year are formed by the pre-existing ones, and nourished 

 by the juice which descends, 



If these conclusions can be maintained, they may seem to 

 bear him out in dissenting from the theory of M. Petit Thouars 

 with respect to the growth of exogenous trees ; but I must 

 admit that they cannot be allowed to set aside the direct evi- 

 dence which the latter botanist has brought together to prove, 

 that fibres actually descend from the branches in certain endo- 

 genous trees, and penetrate between the rind and the old 

 Avood. 



Are we, then, to suppose the mode of growth in these two 

 classes of plants to proceed upon a different principle ? or may 

 we reconcile the two apparently conflicting statements, by sup- 

 posing fibres to be sent down from the buds, and to constitute 

 the rudiments of the young wood and of the young bark, ad- 

 mitting at the same time that these fibres require the descend- 

 ing sap to supply them with materials for further growth and 

 development I 



Without, indeed, pretending to gainsay the arguments alleged 

 by DecandoUe on the contrary side, I must freely admit, that 

 the more recent observations of Mohl, with respect to the 

 structure of endogenous stems, have afl^"orded some additional, 

 though indirect, confirmation of M. du Petit Thouars' theory* 

 as to the fact that fibres descending from buds, may constitute, 

 in endogenous trees, the woody matter of the stem.* 



The reasons on both sides of the question are ably placed 

 before us in Dr Lindley's Introduction to Botany; and a com- 

 parison of what is there said with Decandolle's remarks in his 

 Physiologic A'^egetale may be recommended to those whoge 

 disposition leads them to institute experiments on Vegetable 

 Physiology, as being well calculated to set before them the 

 doubts which still hang over the subject, and the true points 

 which remain open for investigation. 



Another portion of the volume, from which many may be 

 inclined to dissent, is that which relates to the excretory func- 



* See also the Researches of Gaudichaud, in confirmation of M. Petit 

 Thouars' theory. Paris, 1841. 



