442 CONSERVATIVE ORGANS. [LECT, VIII. 



In general, the lateral rootlets extend in the 

 direction of their axis, and display an uniform 

 structure throughout their length ; but this is oc- 

 casionally altered by the nature of the soil. Thus, 

 on the root of the common Sweet-pea, Lathyrus 

 odoratus, when cultivated in a dry soil, we fre- 

 quently find that the rootlets, instead of extending 

 as fibres, swell and assume all the external charac- 

 ters of knobs; which, however, differ from tubers 

 in being merely reservoirs of nutriment, without 

 containing the germs of future buds. When one 

 of these is dissected, the vessels are seen to origin- 

 ate as in the other rootlets, but they soon divide, 

 and embrace the cellular mass which 

 contains the nutritious matter, of which 

 the knob is the reservoir (see marginal 

 cut, in which a. represents the knob, 

 and b. b. b. the natural radicles). The 

 explanation of the formation of these 

 occasional knobs on herbaceous roots, 

 advanced by Sir J. E. Smith, does not 

 now appear to me so satisfactory as I 

 formerly supposed it to be (page 206) ; 

 for I find that they appear on the roots when 

 " no sudden fresh * supply of food is furnished." 

 It is probable, also, that such a supply, if made 

 to a half-starved plant, the roots of which as 

 yet display no knobs, would occasion the na- 

 tural extension of the rootlets, instead of producing 



