6 WHAT THE PLANT IS MADE OF [chap. 



So far the compounds have only contained carbon, 

 hydrogen, and oxygen, but in all plants there is another 

 group of compounds built up as before with carbon 

 as the centre, but with nitrogen also as part of 

 the fabric. Of these compounds containing nitrogen 

 the most important are the proteins (in older phraseology 

 proteids or albuminoids) — complex bodies containing 

 carbon, nitrogen, hydrogen, oxygen, and smaller propor- 

 tions of sulphur and phosphorus. Perhaps the easiest 

 of these bodies to separate is gluten from wheat flour, 

 but by suitable tests they may be recognised in all 

 plants and in all parts of the tissues. The proteins ate 

 very elaborately constructed bodies, and are either 

 insoluble or not properly soluble in water ; but as a sort 

 of intermediate stage in their building up or breaking 

 down, come certain simpler soluble compounds of carbon, 

 hydrogen, oxygen, and nitrogen, often called amides, 

 though the name is not very correct and it will be 

 simpler to call them rt-proteins. 



Of course the carbohydrates, the fats, the proteins 

 are not the only groups of compounds occurring in plant 

 materials : there are also bodies like the essential oils, 

 which give scent to plants, the resins, the vegetable 

 acids, the bitter and poisonous principles, etc., etc.; but 

 the three groups enumerated constitute by far the 

 greater part of every plant, and on them only depends, 

 at least in its broad outlines, the life of the plant and in 

 it< turn of the animal 



Since the crop starts with the seed, we must begin 

 by finding out how a seed is constructed and what 

 conditions are necessary to make it germinate and grow 

 into a plant capable of yielding seed in its turn. Take 

 some conveniently large seeds — beans and maize form 

 the best examples, though they may be supplemented 

 or replaced by sunflowers and wheat— soak them in 



