142 GROWTH OP PLANTS. 



in height many of the palms outrival forest trees. In 

 the palms the hardest part of the wood is the outer, 

 and the softest the inner. 



GROWTH OF TREES HAVING TWO SEED-LOBES. 



PHILOSOPHICAL botanists have held very different 

 opinions respecting the mode in which trees grow in 

 thickness or diameter, as I shall now detail. 



The difficulty, indeed, of arriving at facts, unconta- 

 minated with theoretical fancies in most of the points 

 of vegetable physiology, is nowhere more strikingly 

 exemplified than in the opinions advanced respecting 

 the growth .of plants, particularly trees, in diameter. 



According to Malpighi, the interior part of the cor- 

 tical tube, or in other words, the inner bark, produces 

 the growth in thickness by successively uniting itself 

 to the wood. 



According to Grew there is formed every year, 

 between the bark and the wood, a layer of vessels, 

 which arises from the inner side of the bark and is 

 converted into a new layer of wood. " Every year/' 

 he says, " the bark of a tree is divided into two parts, 

 and distributed two contrary ways; the outer part 

 falleth off, towards the skin, and at length becomes the 

 skin itself: the inmost portion of the bark is annually 

 distributed and added to the wood." 



M. Parent says the interior portion of the bark is 

 converted annually into wood. 



Hales thinks that the pulp wood (alburnum) or new 

 layer of wood, arises from an extension of the fibres 



