GROWTH OF TREES. 14>7 



equally with the stem. By carefully studying, then, 

 the manner in which the bundles of fibres are multi- 

 plied, it will be seen that the growth takes place in 

 a lateral direction ; a direct consequence indeed of new 

 bundles of fibres forming in the centre of the rays, 

 or that of rays in the centre of the bundles of fibres. 

 The circular layers in this way increase in width. 



With respect to growth in thickness, M. Dutrochet 

 is of opinion, that each layer of new wood is first 

 formed of a thin layer of pith, similar to that in the 

 centre, full of cells, giving birth to vessels which form 

 a pith- tube, so that each successive layer of wood is in 

 reality a pith tube, the pith disappearing in all but the 

 centre "one. The bark, he thinks, grows in a similar 

 way, by means of what he terms the hark pith, con- 

 sisting of a thin layer of cellular tissue. 



M. Dela Hire, and after wards Hales and Dr. Darwin, 

 maintained an opinion respecting the growth of plants, 

 which has lately been warmly taken up by M. Du 

 Petit Thouars, and is adopted by Professor Lindley 

 and by one or two living botanists. Professor Henslow 

 says this " rests entirely upon vague conjecture and 

 hypothetical reasoning ; and it appears to me to be the 

 most fanciful and baseless opinion ever propounded." 

 According to this opinion, when a bud shows itself at 

 the base of a leaf, or on a branch or stem, it follows 

 two opposite movements, one upwards towards the air, 

 the other downwards towards the earth. By the upward 

 movement a new branch is produced, while the down- 

 ward movement gives origin to a great number of new 

 fibres which lengthen out between the bark and the 



