Sh ni ee he Life of the Plant. 169 
we can form some judgment, since we know that its pre- 
sence is necessary for the production of chlorophyll. Plants 
which grow in soil destitute of iron do not become green, 
but remain bleached or etiolated until enabled to do so by 
the addition of iron to the soil; though it is doubtful whether 
chlorophyll contains iron as one of its essential ingredients. 
Since etiolated plants, owing to the absence of chlorophyll, 
are unable to absorb carbon dioxide, they can live only for 
a short time, and soon perish. ‘The salts of iron present in 
the soil afford the necessary supply of this element for 
vegetation. Phosphoric acid appears to stand in a certain 
relation to the production of albuminoids, or at least, 1s in- | 
variably found in association with them ; and in many seeds 
there is a constant relationship between the weight of the 
phosphoric acid and that of the nitrogen contained in them. 
With respect to fotasscum, similar general relationships to 
starch, sugar, and cellulose are not improbable; and it 
is known that the amount of potassium in any part 
of a plant varies with the rapidity and energy of its growth. 
But in what form phosphoric acid and _ potassium 
enter the plant, and what their special functions are, is 
stil unknown. In the case of buckwheat, it has, how- 
ever, been ascertained that food-material destitute - of 
potassium cannot be assimilated or transformed into organic 
substances; and that the chloride, and next to that the 
nitrate, are the salts best adapted for the purpose.  Sz/zczc 
acid, which constitutes the greater part of the ash in many 
plants, as, for instance, the stems of cereals, cannot be 
considered a nutrient substance in the same sense as 
those already described; since it has been proved by ex- 
periment that it is not absolutely necessary to the growth of 
these plants, and its presence appears to be only a favour- 
able condition to their perfect development. Nothing more 
definite is known respecting it. 
If plants are grown in fluids which contain all the sub- 
stances necessary for their life, it is found that these sub- 
