THE ROOT 



59 



Experiment 42. — Do they contain different kinds and quanti- 

 ties OF minerals ? — Test in the same way the fresh, active parts of any 

 kind of ordinary land plant (sunflower, hollyhock, pea vines, (itc), and 

 of some kind of succulent water or marsh plant (Sagittaria, water lily, 

 fern). Do you notice any difference in the amount of water given off and 

 of solid matter left behind ? In the character of the ashes left ? Have 

 you observed in general any difference between the ashes of different 

 woods ; as, for instance, hickory, pine, oak ? Compare with the residue 

 left in Exp. 21 ; would you judge that the residual substances are of the 

 same composition ? 



62. Essential constituents. — The composition of the 

 ash of any particular plant will depend upon two things: 

 the absorbent capacity of the plant itself 

 and the nature of the substances con- 

 tained in the soil in which it grows. But 

 chemical analysis has shown that how- 

 ever the ashes may vary, they always 

 contain some proportion of the follow- 

 ing substances : potassium (potash), 

 calcium (lime), magnesium, phosphorus, 

 and (in green plants) iron. These ele- 

 ments occur in all plants, and if any one 

 of them is absent, growth becomes ab- 

 normal if not impossible. 



The part of the dried substances that 

 was burned away after expelling the 

 water consists, in all plants, mainly of 

 carbon, hydrogen, oxygen, nitrogen, and 

 sulphur, in varying proportions. These 

 five rank first in importance among the 

 essential elements of vegetable life, and 

 without them the plant cell itself, the physiological unit of 

 vegetable structure, could not exist. They compose the 

 greater part of the substance of every plant, carbon alone 

 usually forming about one half the dry weight. Other sub- 

 stances may be present in varying proportions, but the two 

 groups named above are found in all plants without excep- 



4 2 13 5 



Fig. 74. — Water cul- 

 tures of buckwheat, show- 

 ing effect of the lack of the 

 different food elements : 



1, with all the elements; 



2, without potassium ; 3, 

 with soda instead of pot- 

 ash ; 4, without calcium ; 

 5, without nitrates or am- 

 monia salts. 



