CHEMICAL BASIS OF PLANT FORMS 237 
in this connection, and the matter cannot be said to be definitely 
settled. 
I can only enumerate in this connection, without going into 
the subject, the possible sources 1 of the nitrogen supply: — 
1. Organic, nitrogenous matter. 
2. The ammonia of the air, and of the ocean. 
3. The nitrous and nitric compounds formed by combus¬ 
tion and by electric discharges. 
4. Nitrogen fixed in the soil by microbes. 
5. The free nitrogen of the atmosphere. 
6. Mineral nitrates. 
The sap which is continually flowing through living plants 
is a watery fluid, holding in solution mineral matters, gases, 
and organic substances. The root hairs of plants penetrate the 
particles of soil and absorb the moisture from a film which sur¬ 
rounds each particle: this is known as hygroscopic water. In 
thallophytes, the absorption is effected by the cells of the 
thallus, and in epiphytes a membrane invests the air roots 
especially adapted for the purpose. The distribution of water 
takes place — at least to some extent — by the same pro¬ 
cess as its absorption. It passes by osmosis from cell to 
cell, as it passes originally from without into the superficial 
cells of the plant. The direction of this movement is not 
necessarily constant. The proportion of water in each cell 
varies and the tendency to establish a fluid equilibrium will 
cause a current towards those tissues which are deficient. These 
statements apply equally to gases and other substances held in 
solution, which are needed for the continuance of the chemical 
and physical changes going on in the living cells of different 
parts of the plant. 
The changes are more active for different substances in dif¬ 
ferent parts of the plant. The mineral substances absorbed by 
the roots pass up to the leaves, where they are concerned in the 
constructive metabolism going on in those organs. The pro¬ 
ducts of these processes pass from the leaves to parts of the plant 
which are actively growing, and where plastic material is re- 
1 “The Economical Aspect of Agr. Chemistry.” By H. W. Wiley. Proc. 
A. A. A. S. y xxxv, 1886. 
