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, etc.), 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. Ss she 
The part of the dried substances that = Fic. 74. —Water cul- 
was burned away after expelling the le a ies te 
water consists, in all plants, mainly of different food elements: 
carbon, hydrogen, oxygen, nitrogen, and 2 ae Lee 
sulphur, in varying proportions. These a pao hrderae ea 
five rank first in importance among the 5, without nitrates or am- 
essential elements of vegetable life, and ™™# ents. 
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- 
