196 
Report of ExperimetUs on the 
before, it would appear that the prevalence of the insect, and its 
consequent injury to the plant, is the result, rather than the original 
cause, of the diseased condition. In fact, it is probable that the 
success of the attack of the insect upon the plant, may be mainly 
due to the weak or unhealthy condition of growth of the latter ; 
and that, had the plant been perfectly vigorous, the insect would 
not be so freely developed, or its injury would at least be more 
successfully resisted. 
ExcremcTiiitious Matters. — There is evidence of various kinds 
to show that plants give out certain substances by their roots 
to the soil. It is not probable, however, that any mineral 
constituents which may be so rejected during the growth of 
one Clover-crop, are prejudicial to the growth of a similar crop 
on the same land for a number of years to come. If the failure 
of the Clover-plant, when repeated too soon upon the same land^ 
be due at all to the excrementitious matters left by the former 
crop, it is much more probable that the injury is in some way 
connected with the organic matters which have been rejected. 
Unfortunately, we are not yet able, by the aid of chemistry, to 
distinguish those organic compounds of the soil which are con- 
vertible into the substance of the growing plant, and those 
which are not so. Nor do we know liow far the excreted 
organic matters may be necessary complimentary products 
in the formation of some of the essential constituents of the 
plant. Experience teaches us that when a crop of Clover 
is eaten by sheep folded upon the land, animals dislike the 
growth which immediately succeeds. It might be inferred, 
therefore, that, in such a case, the plant had taken up from the 
soil, certain matters which it ha'd not finally elaborated. Whether 
these organic substances would, in process of time, be converted 
into living plant-matter, or whether they would wliolly, or in part, 
be rejected as excrementitious organic compounds, to undergo in 
the soil certain chemical changes before being adapted for plant- 
food, we are not able to determine. 
In connexion with this question, of whether or not the failure 
of the Clover-crop be due to the injurious influence of excremen- 
titious organic matters, left by the last crop of the same kind, 
attention may be called to the fact, that in the case of the failure 
in our field experiments, two years of fallow, and one year of 
barley, intervened between the poor crop of Clover in 1855, and 
the almost equally poor one in 1859, A jmori, we should certainly 
be disposed to think, that any deleterious matters left in the soil 
by the Clover-crop of 1855 would, under the circumstances in 
c|uestion, have undergone pretty complete decomposition during 
the three succeeding vears. At the same time, it should be re- 
membered that, in 1852, the plant of Clover suffered very much 
