LECT. IV.] THE ROOT. TUBERS. 163 



Wherever a gem is formed on the tuber, the 

 pith or central part advances to the surface^ and 

 gives a small thread of its substance to the em- 

 bryon ; and this appears to furnish its pith : whilst, 

 in the same manner, the cortical substance, which, 

 although attenuated almost to a pellicle^ yet still 

 surrounds the thread of pith, seems to supply its 

 bark*. The vessels also of the tuber tend to each 

 point where a gem rises on its surface ; and, when 

 this begins to vegetate, they enlarge so much as 

 to become very perceptible to the naked eye ; and 

 acquire a retrograde movement, taking on the 

 functions of absorbents, and conveying the nutri- 

 tious matter deposited in the tuber to the vascular 

 system of the germe for its evolution and support. 



The epidermis of tubers, to whichsoever na- 

 tural order of plants they belong, is utterly devoid 

 of either absorbing or exhaling organs ; and it is 

 this circumstance which fits tubers so admirably 

 for preserving, in an unaltered state, the nutri- 

 ment deposited in them ; the preservation of this 

 in its natural state being absolutely necessary for 



* Vide Plate 1, fig. 4-. A longitudinal slice of a Potatoe : 

 a. the central part, or pith ; b. the cortical part ; c. the change 

 in the position of these parts where a gem is given off; d. the 

 point where the tuber was attached to the runner. It may 

 not, perhaps, be unnecessary to say that the points on the sur- 

 face of a Potatoe, called eyes in common language, are the 

 gems, the rudiments of the expected plants. 



Mi 



