PHOSPHATES IN THE URINE OF THE HORSE. 
195 
derstood when the food supply itself is examined ; for the plants 
upon which the herbivora exist are relatively rich in phos¬ 
phates, and we must explain their slight appearance in the 
urine from some other standpoint. In order to get at the sub¬ 
ject in a comprehensive manner, the relationship of the phos¬ 
phates to the food supply should be noticed. 
The Relation of Phosphates to Plants .—Biologically plants 
are classed with living beings; they possess vital properties ; 
they possess anabolic and katabolic functions ; they assimilate 
and disassimilate; their tissues are built up and the waste ma¬ 
terial is removed, fundamentally, in a way quite comparable to 
that in higher forms ; they have the power of elaborating the 
inorganic material derived from the soil and building it up into 
new organic or protoplasmic combinations. 
The phosphates with which the plants are directly con¬ 
cerned are four in number, namely : the phosphate of potas¬ 
sium, the phosphate of lime, the phosphate of magnesia, and the 
phosphate of iron. 
The earth is especially rich in the phosphates of iron and 
alumina ; phosphates of lime and magnesia are found in quite 
variable quantity and sometimes in extremely slight amounts ; 
the phosphate of potassium is not normally present, and the 
slight quantity found in some analyses is derived from the 
organic debris undergoing decomposition in the soil. The 
phosphate of alumina, quite generally abundant, does not seem 
to be made use of as such by the plants. In order to be utilized 
it must apparently undergo decomposition in the soil, ad then 
resulting phosphoric acid forms new compounds, such as the 
phosphates of potassium, lime, and magnesia. 
The phosphate of potassium, although scarce in the soil, 
exists in considerable amount in the seeds of the plants. It is 
readily soluble in water. 
The phosphates of lime and magnesia are insoluble in water 
alone; but they are easily dissolved in water containing carbonic 
acid, and the moisture of the soil is generally favorable for this 
purpose. The phosphates of potassium, lime and magnesia, 
