3G THIRD REPORT 183". 



in the stem of Monocotyledons there is only one surface of in- 

 crease, namely, that on the inside ; and hence he concludes 

 that such plants have only a cortical system, and consist of 

 bark alone. It must be obvious that there are too many ana- 

 tomical objections to this theory to render it deserving of any 

 other than this incidental notice*. 



The cause of the formation of wood has always been a sub- 

 ject upon which physiologists have been unable to agree ; and 

 if the opinions held by the writers of the last century have been 

 disproved, it cannot be added that those of the present day are 

 by any means settled. It is now, indeed, admitted on all hands 

 that wood is a deposit in some way connected with the action 

 of leaves ; for it has been proved beyond all question that the 

 quantity of wood that is formed is in direct proportion to the 

 number of leaves that are evolved, and to their healthy action, 

 and that where no leaves are formed, neither is wood deposited. 

 But it is a subject of dispute whether wood is actually or- 

 ganized matter generated by the leaves, and sent downwards 

 by them, or whether it is a mere secretion, which is deposited 

 in the course of its descent from the leaves to the roots. The 

 former opinion has been maintained in different forms by De 

 la Hire, Darwin, Du Petit Thenars, Poiteau, and myself, and 

 would perhaps have been more generally adopted if it had not 

 been too much mixed up with hypothetical statements, to the 

 reception of which there are in the opinion of many persons 

 strong objections. For example, it has been asserted that the 

 wood of trees is an aggi'egation of the roots of myriads of buds 

 in a state of action, and that consequently a tree is an asso- 

 ciation of individuals having a peculiar organic adhesion and 

 a common system of growth, but each its own individual life. 

 To this view it is no doubt very easy to raise objections, some 

 of which it may be difficult, in the present state of our know- 

 ledge, to answer ; and therefore it is better for the moment to 

 leave this part of the proposition out of consideration, and to 

 confine it to the simple statement that wood is organized matter, 

 generated by the leaves, and sent downwards by them. In 

 support of this it is argued! 1st, That an anatomical examination 

 of a plant shows that the woody systems of the leaf and stem 

 are continuous : 2ndly, That this is not only the fact in exogenous 

 plants, but in all endogenous and cellular plants that have 

 been examined ; so that it may be considered a universal law : 

 ordly. That in the early spring, and for some time after plants be- 

 gin to grow, the woody matter is actually to be seen and traced 



• Achille Rielianl, Xoiivcaiix Elcmens dc hi Boianique, 5me edit. p. 119. 



