CIRCULATION IN PLANTS. 51 



the direction of the streams being indicated by 

 the arrows. Fig. 240 represents the circulation 

 in one of the jointed hairs, projecting from the 

 cuticle of the calyx of the Tradescantia vir- 

 ginica* in each cell of which the same circu- 

 latory motion of the fluids is perceptible. 



§ 7. Excretion in Vegetables. 



It had long been conjectured by De Candolle, 

 that the superfluous or noxious particles contained 

 in the returning sap are excreted or thrown out by 

 the roots. It is evident that if such a process takes 

 place, it will readily explain why plants render 

 the soil where they have long been cultivated 

 less suitable to their continuance in a vigorous 

 condition, than the soil in the same spot w as origi- 

 nally ; and also why plants of a different species 

 are frequently found to flourish remarkably well 

 in the same situation where this apparent dete- 

 rioration of the soil has taken place. The truth 

 of this sagacious conjecture has been established 

 in a very satisfactory manner by the recent ex- 

 periments of M. Macaire.t The roots of the 



* Fig. 239 is taken from Amici, and Fig. 240 from that given 

 by Mr. Slack, Trans. Soc. Arts, vol. xlix. 



t An account of these experiments was first published in the 

 fifth volume of the " Memoires de la Societe de Physique et 

 d'Histoire Naturelle de Geneve," and repeated in the " Annales 

 des Sciences Naturelles," xxviii, 402. 



