158 CONSERVATIVE ORGANS. [LECT. IV. 



these by insects or frost, or the impeding of their 

 functions by disease, as for example the curling of 

 the leaves of the Potatoe plant, is followed by a 

 decrease both in the quantity and in the size of 

 the new tubers, and also by a deterioration of 

 the nutriment deposited in them. 



The runners to which pendulous tubers are 

 attached are bundles of vessels, produced from the 

 bark of the stem, and intended solely to convey 

 the proper juice from the descending vessels to 

 these knobs ; and that there are no returning 

 vessels, or, in other words, that the progression 

 of fluids through these runners is in a direction 

 from the stem to the tubers only, is rendered pro- 

 bable from the following experiment made by Mr. 

 Knight. He divided the runners connecting the 

 tubers with the stem in a Potatoe plant, and 

 immersed both portions in a decoction of Logwood. 

 The colouring matter passed along in both direc- 

 tions, but did not enter the stem, which it would 

 have done had there been any returning vessels in 

 the runners, whilst it was found to have entered 

 the tubers and filled an elaborate assemblage of 

 vessels which are situated between the bark and 

 fleshy parenchyma *. 



The anatomy of tubers displays a considerable 

 difference in the structure of those which belong 



* Phil. Transactions, 1803. 



