NUTRITION 
33 
dissolves carbonates, nitrates, and other salts. The dilute 
solution is absorbed by the root, and the plant thus obtains 
nine of the ten elements that it needs, viz. nitrogen, hydrogen, 
oxygen, phosphorus, sulphur, iron, magnesium, calcium, 
potassium. Silicon, sodium, chlorine and others are usually 
absorbed also, but though often incidentally useful (e.g. silica 
in stiffening grass stems) are not absolutely necessary. The 
tenth element, carbon, is obtained from the carbon dioxide 
of the air. In presence of sufficiently bright light, this gas 
is decomposed by aid of the green colouring matter ( chloro- 
phyll ) in the leaves &c. ; oxygen is given off, and the carbon 
worked up into complex organic compounds, the process 
being termed assimilation . 
Water is continually evaporating from the subaerial 
organs, especially the leaves. This transpiration is made 
good by the current that flows from the root upwards, 
bringing with it to the leaves the mineral salts absorbed 
by the root. The current travels in the wood or xylem of 
the vascular bundles of the plant; the bundles form the bulk 
of most roots and stems, and appear in the leaves as veins 
or nerves. 
Respiration , or absorption of oxygen and evolution of 
carbon dioxide, is carried on in living parts of plants, as in 
animals. 
Assimilation goes on in the leaves and new living sub- 
stance, or protoplasm , is made, which as fast as it is formed 
decomposes into simpler substances, chiefly sugar and amides, 
which are carried away by the phloem or bast of the vascular 
bundles to those parts of the plant where they are required. 
This continual chemical change is termed metabolism, and 
may be divided into anabolism , the building up, and 
katabolism , the breaking down, of complex materials. The 
materials carried away in the phloem are used at once at 
the growing parts or as food by the living cells, or else 
they are stored up as reserves for future use. All seeds 
contain reserves and so do all plants which die down at 
any period of the year or are arrested in their growth by 
cold or drought. Reserves are also made in many cases 
(Agave, many Palms, &c.) to enable the plant to produce a 
great mass of flowers and seeds at some future time. 
No accumulation of the products of the different steps 
\ 7 . 
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