I.] PROXIMATE CONSTITUENTS OF PLANTS 5 



oxygen — and no other ; the difference between them 

 arises from certain differences in the proportions, and 

 particularly in the manner of building up the structure. 

 Moreover there are particular natural agencies by which 

 one class of bodies can be transformed into the other, 

 just as a house might be taken down and rebuilt from 

 the same materials in a totally different style. 



Among the more important 'of these universal 

 constituents of plants we may place first of all the 

 carbohydrates, a great group of substances so called 

 because in them the elements carbon, hydrogen, and 

 oxygen are combined together in the proportions that 

 suggest a combination of carbon with water. The 

 carbohydrate group includes the sugars, so easily 

 recognisable in the sugar-cane and the beetroot, but 

 also detectable in fruits (a raisin is most easily tested), 

 and by special tests in the leaves and other parts of the 

 living plant. Then come the starches, which are readily 

 washed out of flour, potatoes, and many other storage 

 organs of plants ; also certain gums and mucilages, 

 which are closely related to the starches. Finally, we 

 can include the celluloses and fibres, which we obtain in 

 a very pure state in the simple vegetable cells con- 

 stituting cotton and linen, and less pure in other fibres 

 such as hemp and jute, and in wood itself. 



Rather more concentrated than the carbohydrates, 

 i.e., containing still only carbon, hydrogen, and oxygen, 

 but with a higher proportion of carbon, are the fats, 

 oils, and waxes which are present in many vegetable 

 tissues ; but only in the seeds, nuts, and fruits of certain 

 plants is there enough to be squeezed out for commercial 

 purposes. The seed is always a very concentrated 

 storehouse of food for the future plant, and it is in the 

 seed that such concentrated materials as oils and fats 

 are mainly found. 



