LECT. VIII.] STRUCTURE OF HERBACEOUS ROOTS. 445 



end of the first season of their growth, for then 

 the cells are turgid with the secretions. Perennial 

 roots, also, should be dug up before the central 

 part becomes ligneous.; for, as these roots increase 

 in diameter by annual additions to both their cen- 

 tral and cortical parts, in the same manner as the 

 stems of trees, the interior of the central part be- 

 comes every year more and more inert, and ul- 

 timately decays; so that, in employing such roots, 

 when old, even before they decay, the active 

 principle they contain becomes too largely diluted 

 with the inert matter to answer the purposes 

 expected from them as drugs. 



In closing our researches into this part of Phy- 

 totomy, the anatomy of stems and roots, I may 

 observe that the subject has, hitherto, been 

 imperfectly examined; although the fieJd of in- 

 vestigation is very extensive, and the harvest it 

 contains calculated to repay amply the toil of the 

 most assiduous labourer. The improvements which 

 are daily making in the construction of the micro- 

 scope, are likely soon to set aside all the obstacles 

 depending on the minuteness of the parts; and a 

 very little experience is sufficient to make the stu- 

 dent 'expert in the use of this instrument. The 

 best authors to be consulted on the subject are 

 Grew, Anatomy of Plants; Malpighi, Anatome 

 Plantarum; Rudolphi, Anatomic der Pflanzen; 

 Kie&er, Me*moire sur FOrganisation des Plantes; 



