6' Mr. Colvill on the Functions of the Spiral Vessels 



tence, or to its lateral increase. As has been already observed, in 

 the stem of the tree the spiral vessels are all ranged round the 

 centre or pith. * Every one is aware, however, that many ex- 

 amples are furnished by the stems and branches of decayed trees, 

 in which, " even after the heart is become hollow," and the woody 

 part almost wholly removed, the tree still continues to vegetate, 

 and new layers of wood to be annually added for a long series of 

 years, t These facts, I apprehend, demonstrate that the existence . 

 of the spiral vessels in the stem or branch is not necessary either 

 to its preservation, or its lateral increase. 



Hitherto we have spoken of the spiral vessels only, but the same 

 conclusion must be formed with regard to the pith. In the decay- 

 ed trees already adverted to, the pith also is absent, while the tree 

 nevertheless continues to vegetate ; and Mr. Knight having con- 

 trived to abstract from some annual shoots a portion of their pith, 

 so as to interrupt its continuity, found that the shoots were not ma- 

 terially affected by the operation. % 



We are hence entitled to conclude, that subsequent to the for- 

 mation of the annual shoot, when it becomes fitted for the opera- 

 tion of grafting, neither the spiral vessels nor the pith are essential 

 to its preservation or growth. At what period then do they ex- 

 ercise their functions ? Let us trace them from their origin in the 

 bud. 



Buds, according to Sir J. E. Smith, " consist of scales closely 

 enveloping each other, and enfolding the embryo plant or branch."|( 

 But it will be asked, what are the organs of which this embryo 

 plant or branch consists } The author of the article on Vegetable 

 Physiology, in the Library of Useful Knowledge, furnishes this in- 

 formation.§ " The first which can be distinctly recognized is the 

 pith : surrounding this, we next perceive lines, which are the first 

 traces of the spiral vessels of the future branch, running in a di- 

 rection from the centre of the parent branch to the peak of the 

 stem : indications of the separation of the cellular matter into scales 

 are next perceptible." In the bud, then, the pith, medullary 

 sheath, and cellular integument already have their existence. But 

 the existence of the pith can be traced anteriorly to the existence of 

 the spiral vessels. The pith, therefore, has not been produced by 

 the agency of the spiral vessels, neither has the cellular integu- 

 ment been so, because it is part of, and connected with, the cellular 

 integument of the parent plant. But if the spiral vessels have not 

 been the agents of the production of any of the other organs of the 

 bud, it is a just inference that, while in the bud, the functions of 



• Smith's Introduction to Botany, C. 8. p. 38, &c. 



t Ibid. C. 7. p. 32 ; Keith, B. iv. C. 3. Sec. 2. 



:J: Philosophical Transactions, 1801. 



II Smith's Introduction, C. 14. 



§ Lib. of Useful Knowledge, Veg. Phys. p. 19. 



