THE SEED AND GERMINATION 31 
Sugar differs from the other two in being soluble 
and in having more oxygen and hydrogen in its com- 
position. Sugar, as we shall find later, is abundant in 
most fruits as well as in many roots and sueeulent 
stems. Starch is insoluble in water and is present in 
most seeds and underground stems. Cellulose forms 
the bulk of all wood, while good filter paper as well 
as cotton wool are pure cellulose. The simpler tests 
for this substance are not always reliable, but the 
following can usually be applied with success :— 
Soak the material to be tested, say cotton wool or 
filter paper, in weak iodine solution and then place on 
it a drop of strong sulphurie acid. A blue colour 
appears, which almost immediately changes ‘to black. 
Where this blue colour shows itself we may be sure 
that cellulose is present, but where it is not seen we 
cannot be so sure that it is absent. 
Oils, which, like carbohydrates, are composed of 
carbon, oxygen, and hydrogen, but in different pro- 
portions, are plentiful in many seeds. In seeds such 
as the castor bean the presence of oil may be shown 
by rubbing the broken surface of the seed on a piece 
of tissue paper. A transparent greasy mark shows 
that oil is there. Where not plentiful, the oil may be 
extracted by crushing and soaking a number of seeds 
in ether in a tightly corked test-tube. The ether 
dissolves out the oil: after allowing the tube to stand 
for some hours, pour off the ether without disturbing 
the sediment of crushed seed, and allow it to evaporate 
in a shallow dish. When the ether has disappeared 
drops of oil will remain. 
Proteins are found in all plants. Carbohydrates 
and oils contain carbon, oxygen, and hydrogen. The 
same three substances are found in proteins, but in 
addition, the gas nitrogen, the mineral sulphur, and, 
as a rule, the element phosphorus enter into their 
composition. Proteins then, are the nitrogenous 
