ON I'l^ANT ("oMrosrnoN and .m.\ni imal i;k<m iIxK.mkn'is. 11!) 
of fixed silica, which sometimes in the roots amounts to 78 per cent, of 
the Aveight of the ashes, and 50 per cent, of the weight of the leaves and 
stalks. Consequent, possihiy, upon experiments too rapidly carried out, 
the assertion has heen thought justified that silica, from the elementary 
point of view, played hut an unimportant part. This assumption was 
probably a too hasty one. Silica is rendered soluble in the soil and 
enters into the plant, especially during the primary phase of develop- 
ment, a phase which may be termed the rooting period. At this time it 
represents in Wheat i'.- of the weight of the leaves, and already three-fourths 
of this silica have become insoluble. In proportion as the plant matures, 
the relative weight of the silica diminishes, although its total weight in- 
creases, and, curiously enough, the soluble portion of silica, that which 
appears to be combined with organic matter, increases. There is a 
certain relation between the assimilation of the hydrocarbonised and 
nitrogenised matters, and the fixation of the silica. Later experiments 
will determine this point, which can only have a scientific interest, silica 
being generally abundantly present in soils. 
Magnesia. — The part played by magnesia in the alimentation of plants 
may be compared to that of lime ; its action, however, is far less im- 
portant. The examination of the tables of crop-requisites shows that for 
plants which are not grown exclusively for the production of fruits or seed 
the importance of the presence of magnesia is diminished. It is found 
in the soils used for horticultural purposes, associated either with 
lime or silica. The soils which are richest in magnesia are derived 
from the disintegration of dolomitic limestones (carbonate of magnesia). 
Precisely as wuth lime, it is only that portion of the magnesia which can 
be extracted from the soil by weak acids that can be regarded as useful. 
Jro;?. Oxide of iron is plentifully distributed in the majority of soils. 
This element, which is certainly indispensable to plant life, does not, 
however, play so important a part as has long been attributed to it. It 
was thought that iron took an altogether special part in the formation of 
chlorophyll. Recent research has demonstrated that frequently the 
chlorotic plants contain more iron than others, while previously the 
general idea was that in such plants iron was absent. The work of dis- 
solution of the iron in the soil is effected by the action of dissolved carbonic 
acid. Soluble phosphate of lime forms with the humus, the alumina, and the 
oxide of iron, various organic phosphatic lacs, which very slowly furnish 
to the roots of plants the phosphorus and iron which they may need. 
Generally speaking, nine-tenths of the good results attributed to the sul- 
phate of iron employed as manure occur in calcareous or clayey calcareous 
soils. The sulphate of iron, coming into contact with the carbonate of 
lime, forms sulphate of lime, which, finely divided, acts as a veritable 
plaster. 
Chlorine and Sodium. — All plants contain chlorine ; there is also 
found chloride of sodium and chloride of magnesium in almost all soils. 
The physiological property of chlorine does not seem to be clearly deter- 
mined. It has, however, been ascertained that if the chlorine entered the 
plants in the form of chloride of sodium, the weight of soda recovered 
from the tissues was not equivalent to that indicated by the formula of 
chloride of sodium. There is a transformation there by double decomposi- 
