SECT. II. COMPOSITE ORGANS. 



taking contrary directions ; the outer portion extend- 

 ing towards the circumference of the plant, and 

 forming the new layer of bark ; and the inner por- 

 tion receding towards the centre and forming the 

 new layer of wood. This hypothesis is preferable to 

 Malpighi's, inasmuch as it accounts for the new layer 

 both of wood and bark ; and perhaps no hypothesis 

 whatever is more conformable to fact. Hales was of 

 opinion that the new layer of wood is formed from, 

 the layer of the preceding year, both by means of the 

 dilatation of the horizontal vessels of the former layer, 

 and also of the extension of the longitudinal tubes. 

 The opinion was founded upon the phenomena of a 

 tumor that seemed evidently formed from the wood ; 

 whence he inferred that the new layer is also formed 

 from the wood. But as it could never have been 

 deemed logical from the phenomena of the diseased 

 action of a part to infer the mode of procedure in 

 the sound action of the whole, the hypothesis does 

 not appear to have met with many advocates. Lin- According 

 naeus thought the new layer of wood was formed ^s?" 

 from the pith, which is absurd, because the opinion 

 goes to the inversion of the very order in which the 

 layer is formed, the new layer being always exterior 

 to the old one. But according to the most general 

 opinion, the layer was thought to be formed from a 

 substance oozing out. of the wood or bark first, a 

 limpid fluid, then a viscid pulp, and then a thin 

 layer attaching itself to the former ; the substance 

 thus exuding from the wood or bark was generally 



