INTRODUCTION. 17 



tables, the turnip and carrot, tubers differing 

 only in form, are examples. If the seeds of these 

 plants be sown when Nature intends they should, 

 viz. when they are ripe, the plants are half 

 formed in the autumn of the first, and perfected 

 in the summer of the following year. It is said 

 that the size of these bulbs or tubers are en- 

 larged by the action of their system of foliage ; 

 this geneYa,tiug and throwing down supplies which 

 are destined to swell the bulbs and perfect the 

 seeds in the ensuing season. For this purpose 

 it is said the leaf-stalks are provided with de- 

 scending sap-vessels in the first, though they 

 must be unnecessary in the second, year of their 

 growth. The knowledge of this remarkable 

 change in the petiole of the leaves is a discovery 

 of microscopic examination. To naked practical 

 eyes this formation is never visible ; and when 

 this doctrine is practically considered, we must 

 conclude therefrom, that all individual plants, 

 having the largest growth of leaves, must also 

 have the largest roots. But the reverse of this 



