34 
METABOLISM 
in anabolism takes place, the upward progress to protoplasm 
being gone through very rapidly, but with katabolism it is 
different and accumulations of the products occur. The 
first downward step is the decomposition of protoplasm into 
pi'oteids (complex organic bodies containing carbon, hy- 
drogen, oxygen, and nitrogen) ; the next is to amides 
(crystallisable organic bodies, containing the same elements; 
the commonest is asparagin); then, by elimination of nitrogen, 
the carbohydrates (bodies containing carbon, hydrogen and 
oxygen in the proportions C^Ho^O^; the chief are starch, 
cane-sugar, grape-sugar or glucose, mannite, inulin, and 
cellulose), the oils or fats (containing C, H, O), tannins , and 
other bodies, are formed. A further decomposition may 
take place, in respiration (which is the expression of kata- 
bolism), giving as final products carbon dioxide and water. 
All the products mentioned so far are capable of being used 
again in the metabolism of the plant, but there are others 
which are apparently waste-products. Only rarely are these 
excreted at the surface ; usually they are stored in the cells 
or in special cavities or passages. Some of these bodies 
are of economic value, eg. resins, alkaloids, ethereal oils, 
wax, &c. 
In forming reserves the plant always stores up both 
nitrogenous and non-nitrogenous materials. The former 
take the shape of proteids in seeds ; in other places they 
may be proteids or amides. In seeds the latter are usually 
stored as starch or cellulose ( starchy seeds) or as oils (oily 
seeds) ; in other places other forms occur. In germination 
ferment-action causes the decomposition of the reserves and 
their transmutation into sugar and amides, in which forms 
they travel through the plant. 
Growth, usually implying increase in size and weight, 
but better defined as permanent change of form, is a feature 
of all living plants and organs. In those with which we 
have to deal, growth is only general in the whole plant in 
the earlier embryonic condition, and subsequently becomes 
localised in certain growing points , eg. at the tips of stems 
and roots, and growing layers , eg. the cambium by whose 
means stems and roots grow in thickness. The new tissues 
and organs formed in these places expand to their full size 
and then usually cease to grow. 
