LETTER XIII. 



Truth proposed is much more easily perceived, than without 

 such proposal it is discovered." Archbishop Secker. 



March 29, 1855. 



My Dear Sons, 



1 . I proceed, as proposed in my last letter, to con- 

 sider the evidence in favour of the assumption that 

 the vroody tissue evolved from the Cambium-layer is 

 of the nature of roots. 



2. And the first consideration which I shall urge is 

 this, — that, whenever the circumstances of the Cam- 

 bium are such as to admit of it, the tissue developed 

 from this layer palpably assumes the character, puts 

 on the aspect, and grows after the manner of roots. 

 Take, for example, the process of slipping." You 

 cut off the upper part of the young shoot of a willow, 

 to which is attached a bud, and you plant it in the 

 ground. In due time the bud sprouts, and from all 

 that part of the Cambium which is in immediate con- 

 tact with the soil, genuine roots are given off, such as 

 will be given off from the Cambium in the parent 

 tree, at the tips of the last year's roots, and nowhere 



