74 BOTANY 
help the leaves in their work. In the soft mud, more- 
over, there is no special need to protect the tender 
growing point, and henee, in the mangrove, the root- 
cap has been dispensed with. 
In many orchids, such as the native dendrobium 
and earina, the roots do not bury themselves in soil, 
but, clinging to some tree trunk, absorb, by means of 
a surface layer of spongy tissue, any particles of water 
that may drip upon them. 
CHEMICAL CONSTITUENTS OF PLANTS. 
All plants are built up of organic compounds 
formed from the inorganic substances derived from 
earth and air. To analyse the plant and identify the 
various elements that go to the building of these 
compounds requires a knowledge of chemistry far 
beyond the scope of this book. We shall therefore 
merely give certain more or less rule-of-thumb methods 
of testing for their presence and indicating their 
composition. 
Carbohydrates. For starch, sugar, and cellulose 
the tests have already been given. In the dahlia and 
Jerusalem artichoke we find stored inulin, another 
carbohydrate which does not respond in the ordinary 
way to either iodine or Fehling’s Solution test. Crush 
a dahlia root with a little water, filter off the liquid 
into a test-tube, and add methylated spirit. A dense 
white precipitate shows the presence of inulin. Now 
add iodine and the precipitate becomes yellow. 
Oils and their composition have received attention 
in an earlier chapter. 
Proteins, such as gluten and legumin have already 
been dealt with. Many proteins contain, in addition 
to the carbon, oxygen, hydrogen, nitrogen, and sulphur 
already mentioned, varying quantities of phosphorus. 
By strongly heating in a test-tube, the gluten of wheat 
or the cotyledon of a bean seed, the presence of sulphur 
