180 HOW CROPS GROW. 
ash of wood of Pinus sylvestris 18.2°|, Mn, O,, and 3.5°|, 
Fe, O,. In ash of the seed of colza, Nitzsch found 16.1°|, 
Mn, O,, and 5.5 Fe, O,. In case of land plants, these high 
percentages are accidental, and specimens of most of the 
plants just named have been analyzed, which were free 
from all but traces of oxide of manganese. . 
Salm-Horstmar concluded from his experiments that 
oxide of manganese is indispensable to vegetation. Sachs, 
Knop, and most other experimenters in water-culture, make 
no mention of this substance in the mixtures, which in 
their hands have served for the more or less perfect devel-. 
opment of a variety of agricultural plants. Birner & 
Lucanus have demonstrated that manganese is not needful 
to the oat-plant, and cannot take the place of iron. (Vs. 
St., VIL, p. 48.) 
Is Chlorine indispensable to Crops’—What has 
been written of the occurrence of soda in plants ap- 
pears to apply in most respects equally well to chlo- 
rine. In nature, soda, or rather sodium, is generally 
associated with dblorine as common salt. It is most prob- 
ably in this form that the two substances usually enter 
the plant, and in the majority of cases, when one of them 
is present in large quantity, the other exists in correspond- 
ing quantity. Less commonly, the chlorine of plants is in 
combination with potassium exclusively. 
Chlorine is doubtless never absent from the perfect agri- 
-cultural plant, as produced under natural conditions, though 
its quantity is liable to great variation, and is often very 
small—so small as to be overlooked, except by the careful 
analyst. In many analyses of grain, chlorine is not men- 
tioned. Its absence, in many cases, is due, without doubt, 
to the fact that chlorine is readily dissipated from the ash 
of substances rich in phosphoric, silicic, or sulphuric acids, 
on prolonged exposure to a high temperature. In the 
later analyses, in which the vegetable substance, instead 
of being at once burned to ashes, at a high red heat, is 
