GROWTH. 127 



which may vary in the two stems, and that there is no 

 more necessity that they should be produced in the 

 same manner than that they should be disposed in the 

 same manner. 



193. In respect to the want of uniformity in the 

 woody layers of trees of tropical countries, we may 

 remark that the theory of Du Petit Thouars is on 

 a level with the other in regard to an explanation, for if 

 by the one it is made to depend upon a greater develop- 

 ment of buds on certain sides of the tree than on others, 

 and therefore in those places a greater transmission of 

 roots downwards from their bases, so by the other it may 

 depend upon a greater transmission of formative juices 

 downwards in the bark, instead of fibres free from it. 



194. But it must be candidly allowed that the circum- 

 stances adduced as proof of wood not being formed in 

 those cases in which rings of bark have been removed 

 from a stem is owing to an interruption of descending 

 juices through the bark, might be looked upon as equally 

 corroborative of an interruption of descending fibres 

 from the buds being the cause. The wood when first 

 deposited is of a yellowish white colour, and receives 

 the name of Alburnum ; as it gets older it has a more 

 decided colour, and becomes harder, and towards the 

 centre of the plant is called Duramen, or heart wood. 



195. Malpighi seems to have been the first who stated 

 that the number of zones of wood seen in a plant was 

 equal to that of the years it had lived. 



196. This mode of reasoning is now known to be 

 open to much fallacy. In our own climate, and other 

 temperate and cold ones, where we are aware that the 

 deposition of one zone of wood takes place annually, 

 and only one, we may come near the mark ; but even in 

 France it was found by Adanson that some Elms which 

 were known to have been planted for 100 years in the 

 Champs Elysees, varied in the number of their zones in 

 different specimens from 94 to 100 ; but which has 

 been explained away by supposing that when planted 

 they were not all of the same age. 



