Comparative Physiology. 561 



allj even the most nourishing, have a limit beyond Avhich they 

 are in excess and prove hurtful, that plants must possess a power 

 of excretion ; and, as many of these substances are not easily 

 evaporated, that they must be excreted by the roots. Liebig 

 (who takes the excretionary power as undoubted), in his Agri- 

 cultural Chemistry, in the excellent chapter on alkalies, notices 

 that every plant has certain kinds of acids and alkalies that 

 it prefers, but, in certain situations, is found to change these 

 for others, when the proper kinds are not contained in sufficient 

 quantity in the soil. Marine plants possessing soda in their 

 natural situations, are found to have the soda replaced by potash 

 in soils deficient of the former and abounding in the latter. 

 Trees generally found to have their acids united to jootash 

 have, in saline soils, been found to have the potash replaced by 

 soda. In some soils, v^here there was a deficiency of matter to 

 furnish materials for the ordinary vegetable acids, they have 

 been found replaced by mineral acids, as the sulphuric, &c. 

 All these instances point out that, where the normal acids and 

 bases are found in sufificient quantity, all others not suitable, 

 though capable of passing the pores, and certainly carried 

 up in the ascending sap, as proven by their being sometimes 

 assimilated, are yet, when the normal substance is to be had, 

 undoubtedly rejected and excreted. Even the normal sort, if 

 in excess, must also be excreted. Liebig notices that the quan- 

 tity of potash in wheat was found to decrease much in amount 

 as the plant came to maturity. Schlieden (who has doubts as 

 to plants possessing the faculty of excretion by the roots) says 

 this is owing, not to the wheat parting with its potash, but to 

 less being assimilated by the ripening plant ; and, as the other 

 parts increase more than potash at this period, the proportion is 

 less, though the original quantity is not diminished. This, 

 however, does not alter the case ; for, if the water absorbed 

 contains nearly the same quantity of potash as before, which is 

 generally likely to be the case, if less is assimilated more must 

 be rejected. No doubt the young growing parts possess most 

 unassimilated alkalies in the circulating sap ; but the quantities 

 of these contained in a soluble state in the water of the soil are 

 generally much more than needed. Professor Johnson describes 

 potash, soda, &c., as carriers of organic substances, dissolving 

 and uniting to the vegetable substances found in the soil, Avhich 

 they part with, giving them up to be assimilated, and are again 

 excreted to dissolve fresh portions and be again absorbed. On the 

 whole, the general opinion seems to be, and is most likely to be 

 correct, that many substances are taken up besides those needed, 

 and must be rejected, as also needful substances when in excess ; 

 and tliis is likely the cause of what has been asci'ibed to selec- 

 tion. It has been also said that, as leaves reject nitrogen, so 



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