8 Mr. Colvill on the Functions of the Spiral Vessels 



and plumula, all undergo a similar development ; and while it will 

 be recollected that the spiral vessels are to be found in each of 

 these organs, it must not be forgotten that these are the only parts 

 of the plant in which the spiral vessels have hitherto been traced. 

 It may be objected, however, that the root undergoes a longitudi- 

 nal increase, but that, as is already admitted, no spiral vessels are 

 to be found in it. True ; but there are two modes by which the 

 augmentation of the different parts of a plant is effected. " The 

 root is elongated merely by the extremity,"* but " the develop- 

 ment of the shoot from the stem is not effected in the same manner 

 as that of the root, — by additions to the extremity only, — but by 

 the intro-susception of additional particles throughout its whole 

 extent, at least in its soft and succulent state, the longitudinal ex- 

 tension diminishing in proportion as the shoot acquires solidity, and 

 ceasing entirely when the wood is perfectly formed ; though often 

 continuing at the summit after it has ceased at the base."t This 

 manner of increase is not peculiar to the stem, but may likewise 

 be observed in all the other parts in which the spiral vessels can 

 be traced. Strictly speaking, therefore, the spiral vessels are not 

 the organs by which every increase in length is effected, but mere- 

 ly the organs by means of which the peculiar elongation of the 

 stem, leaf, flower-stalk, &c. is produced. 



The functions of the pith must not, however, be lost sight of. 

 This organ is to be seen in its full vigour in the bud, and in the 

 young growing stems or branches, it is " a tolerably firm juicy sub- 

 stance," X but " becoming dry before the first layer of wood is per- 

 fected, except near the final bud, or where branches are given off, 

 in which places it retains its moisture." || Mr. Knight's experi- 

 ment has shown, that the pith is not essential to the existence of 

 the young shoot or stem, and the circumstance of its becoming dry 

 so soon as the elongation of the plant ceases, demonstrates that at 

 this period its functions are terminated. 



But what are these functions ? It has been already shown that 

 the pith is not necessary to the subsequent existence of the stem. 

 It has been likewise shown that the existence of the pith is not 

 necessary to the production of the annual ligneous and cortical 

 layers. Its functions, therefore, must be confined to supplying 

 nourishment to the bud and stem during their elongation. 



It is not my intention, in this essay, to enter upon any investi- 

 gation of the manner by which the elongation of the bud into a 

 stem is effected by the medium of the spiral vessels and pith. My 

 sole object is to demonstrate, if not the certainty, at least the very 

 great probability, that exists, that such is the use of these organs. 



• Keith, B. iv. C. 4. Sec. 4. Sub-sec. 1. 



t Keith, B. iv. C. 4. Sec. 4. Sub-sec. 2. See also De Candolle, B. ii. C. 4. 

 Sec. 2. Alt. 4. where this subject is fully treated. 

 4: Smith's Introduction, Chap. vii. 

 II Lib. Useful Knowledge, Veg. Phys. p. 12 and 18. 



