CHAP. II.] CHEMISTRY, ETC., OF PLANT LIFE. 55 



non-volatile compounds of the plant that remain aa a 

 •white ash after the plant has been burnt. 



In addition to the above, various other elements occur 

 in certain plants, but their connection with nutrition has 

 not yet been proved ; amongst these are, Manganese 

 (Mn), Sodium (Na), Lithium (Li), Iodine (I), Bromine 

 (Br), Silicon (Si), and in rarer cases some of the metals, 

 as Aluminium (Al), Copper (Cu), Cobalt (Co), Barium 

 (Ba), Zinc (Zn), Nickel (Ni), Strontium (Sr). 



Chlorine (CI) has so far been proved by experiment 

 to be an indispensable food element in the case of one 

 plant only, the Buckwheat. 



Direct experimental cultures with plants have proved 

 that they can be grown and perfectly nourished if sup- 

 plied with the requisite elements in compounds suitable 

 for absorption ; the same experiments have also proved 

 that certain substances often met with in their ash are 

 not necessary for their growth, inasmuch as equally 

 healthy plants can be produced when such elements are 

 intentionally kept away from their food supply. Such 

 substances, which usually occur in very small quantities 

 in plants, may be considered as having been absorbed 

 by the plant along with the necessary food. 



The above food constituents are not taken into the 

 plant as elements, but as compounds ; carbon, for 

 example, is obtained from carbonic dioxide ; nitrogen, 

 although so abundant an element in the atmosphere, is 

 never assimilated in the free form, but as nitrates or 

 compounds of ammonia that are soluble in water ; hydro- 

 gen is obtained from the decomposition of water ; the 

 remaining substances are taken by the plant in the form 

 of compounds soluble in water ; thus sulphur is obtained 



