Theory of the OHgin of Wood. 373 



the views of Du Petit Thouars, is the existence of woody ex- 

 crescences in the bark of trees. They present themselves most 

 frequently on the beech in the form of a nodule projecting 

 from the bark of the trunk of the tree. On examining them 

 it will be found that they have no connexion with the wood of 

 the tree, and consist of several layers of contorted woody tis- 

 sue enveloped in a bark of their own, consisting of liber and 

 cellular integument. They are of all sizes, from those com- 

 mencing existence not bigger than a pin's head, to some that 

 attain the size of an orange. The smallest appear to consist of 

 nothing but cellular tissue ; but as they increase in size a little 

 spot can be seen in the centre, which appears to be the com- 

 mencement of the formation of woody tissue. As they increase 

 an obvious separation takes place into a central nucleus of 

 woody tissue, and an enveloping integument consisting of 

 woody and cellular tissue. In the spring, when the cambium 

 is found to exist between the bark and wood of the tree, it is 

 found in these excrescences ; the nucleus of wood is then 

 easily removed from its bark, and frequently falls out when 

 the bark is broken. On some of them, and especially the 

 smaller ones, buds are observed at the beginning of the year, 

 but these seldom produce leaves. They are more abundant 

 on the beech than any other tree, but are frequently met with 

 on the elm, oak, walnut, crab, sycamore, &c. On cutting into 

 the nucleus several layers of wood can be distinguished, which 

 by maceration can be separated from each, indicating un- 

 doubtedly their yearly growth. 



Sometimes a large number of these nodules are developed 

 together, forming one large knob : this occurs particularly in 

 the elm and acacia, the wood of the latter of which is fre- 

 quently used for ornamental cabinet-making, on account of 

 the beautiful markings which the central points and the con- 

 centric lamellae of the nodules afford. These large knobs sel- 

 dom develope branches, and although in these cases they lie in 

 contact with the wood of the trunk of the tree, yet a distinct 

 separation can be observed between the wood of the knobs and 

 the wood of the tree. This separation is so evident in many 

 cases, that it is obvious the wood of the knobs had not a com- 

 mon origin with the wood of the trunk. The bark of the 

 knobs and the trunk are continuous. 



