132 



LETTERS ON TREES. 



an elongation in the vertical direction (on one side or 

 on two opposite sides), and thus elongating become 

 fusiform or spindle-shaped fibres. These cells have 

 likewise a peculiar tendency to the production within 

 their substance, or on their inner surface, of a consoh- 

 dating deposit, to which they owe the tenacity and 

 toughness which gives to wood its value. Their elon- 

 gation, according to Dr Carpenter, does not seem to 

 be the result of pressure, but of the unequal nutrition 

 of different parts of the cell-wall.* Whether each 

 individual cell is drawn out to any considerable extent, 

 or whether a number of them coalesce to form a single 

 fusiform fibre, does not yet appear. Neither is it 

 known whether the individual fibres join end to end 

 together, and so form continuous threads of great 

 length. But we shall see by and by that some such 

 union and coalescing of cells and fibres must needs 

 take place. And we shall see also that there is a good 

 deal more in this part of the process of transformation 

 than has yet been unfolded even in our more recent 

 treatises on vegetable physiology. Certain it is that 

 the whole account of the matter is not ended by saying 

 that the tissue in question is formed in situ and by an 

 elongation of the cells. 



15. That this tissue is really of the nature of roots, 



* Principles of Physiology, General and Comparative, 3d ed., 

 p. 99. 



