10 THE FLOWERING PLANT. 
head is obtained in two ways—(r1) by chemical analysis of plants ; 
(2) by cultivating them in various solutions, and determining 
which are most suitable. 
The essential elements (always found on analysis, and compounds 
of which form a suitable food-solution) are the following :—Carbon, 
hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 
calcium, magnesium, and iron. The first six actually make up 
the plant-body, while the remaining four have a beneficial in- 
fluence upon the vital processes. For example, chlorophyll can- 
not be formed when tron is absent. Carbon is obtained from 
carbon dioxide; hydrogen from water and ammonia or its com- 
pounds ; oxygen from water and numerous salts; nxtrogen from 
either ammonia and its salts, or else nitrates; sulphur from sul- 
phates ; phosphorus from phosphates; potassiwm from various 
compounds, especially chloride; calciwm and magnesium as sul- 
phate, phosphate, nitrate, and carbonate; dron from numerous 
‘compounds. 
Reception of food is followed by assimilation. That is to say, 
the simple food-substances are built up step by step into the 
complex compounds which constitute the plant-body. The jist 
step consists in the formation of non-nitrogenous organic matter 
from carbon dioxide and water, with liberation of oxygen. The 
equation 
CO, + H,O = CH,O + "Gs 
carbon dioxide and water give non-nitrogenous matter and oxygen 
roughly represents this. It must not, however, be supposed that 
the liberated oxygen is all derived from the carbon dioxide, though 
half must be, while the other half comes from the water. The 
oxygen in question passes off, at any rate to a large extent, into 
the surrounding medium. It is most essential to remember that 
this process has nothing whatever to do with the breathing of plants. 
This first step in assimilation, which bridges the gap between 
inorganic and organic compounds, can only be effected by chloro- 
phyll-bearing protoplasm in presence of light. It has been 
already mentioned that kinetic energy is liberated when chemical 
compounds break down. Conversely, kinetic energy is converted 
into potential when chemical compounds are built up. The neces- 
sary kinetic energy appears to be obtained by the chlorophyll 
from light-rays. 
When organisms contain no chlorophyll, they cannot live on 
simple inorganic compounds only; hence the complex nature of 
animal food. ‘There is also no evolution of oxygen. 
The first visdble product of assimilation is starch, which has 
a chemical composition closely allied to that of cellulose. The 
ultimate product of assimilation is protoplasm. 
