LECT. IV.] THE HOOT. TUBERS. 155 



conjecture which is not improbable, that the final 

 cause of the length of the runners is to place the 

 tubers at a convenient distance from the parent 

 plant and from each other, in order that the new 

 plants may enjoy the advantages of a fresh and 

 unexhausted soil. 



The Potatoe, which affords the best exemplifica- 

 tion of this species of tuber, has many gems on 

 its surface, all of which shoot up into stems ; and 

 from each, besides the runners for the production 

 of new tubers, rootlets are given off for the ab- 

 sorption of that portion of nutriment which is 

 required to be supplied from the soil for the sup- 

 port of^the vegetating plants. 



Such are all the species of tubers which I 

 think it necessary to particularize ; and to one 

 or other of them, every known tuber may be re- 

 ferred. Other species are undoubtedly noticed by 

 authors, under the head of tuberous roots; but 

 these will be found to be varieties only of some 

 of those we have examined : even the granules 

 attached to the roots of the White Saxifrage, 

 Saxifraga granulata, are sometimes quoted* as 

 tubers, although they actually belong to the next 

 of the appendages of the root, which we have to 

 take under consideration, the bulbs. The roots of 

 several species of the Iris tribe have some resem- 



* Keith's Syst. of physiological Botany > vol. i. p. 39. 



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