82 BOTANY 
protoplasm, as well as of all other proteins. Though 
nitrogen forms four-fifths of the atmosphere, ereen 
plants are not able to make use of the free nitrogen of 
the air. It is only through the medium of humus they 
can obtain it. On decomposing, the organic matter of 
the soil sets free ammonia, a gas consisting of nitrogen 
in combination with hydrogen. This unites with the 
oxygen of the air to form nitric acid, and the nitric 
acid combines with some substance like lime or potash 
to form a nitrate. It is in the form of a soluble nitrate 
that the green plant obtains its nitrogen. 
Saltpetre is potassium nitrate, and is one of the 
forms of nitrogen available to the plant. Nitrate of 
soda, or Chile saltpetre, is another compound that 
serves the same purpose, being largely used as a manure. 
There are plants which obtain the bulk of their 
nitrogen from the bodies of insects and small animals 
which they entrap. The sundew (Figs. 54-55) which 
grows plentifully on the poor lands from North Cape 
to Banks Peninsula is a good example of this. The 
leaves of the plant are divided into narrow segments 
and exude a glistening liquid, which, having the 
appearance of nectar, attracts insects. As soon as the 
insect settles, it is caught. by the sticky dew and the 
leaf segments curl over and hold it tight. There is 
then poured on it a juice like the gastrie juice of an 
animal, which dissolves the soft parts. The solution 
is then absorbed by the plant, and the leaf onee more 
opens out ready for another victim. Sundews, because 
of their insectivorous habit,can grow on land poor in 
nitrogen. 
Parasites are plants which do not gather and pre- 
pare food for themselves, but penetrate with their 
roots the tissues of other plants and absorb from them 
the sap containing the nutritive materials they need. 
Plants that are completely parasitic, then, get their 
