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schleiden’s views ox the diseases of cultivated plants. 
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being placed in the same condition as the cultivated plants, are exposed to like influences). The 
question then arises, whether the truly wild plants are subject to diseases properly so-called; whether 
the blights, &c., which we see them suffer from occasionally are not altogether a result of external 
causes, such as peculiar seasons, &c., and, arising from transitory influences, have but a transitory 
duration? This question the author leaves open, his immediate business being with the cultivated 
plants. 
Here the conditions are totally different. Scarcely any one of our cultivated plants is in a natural 
condition; almost all deviate, more or less, from their typical specific form, as defined by the natu¬ 
ralist. The unnatural and excessive development of particular structures or particular substances, 
destroys the equilibrium and lays the plant more open to suffer from injurious external influences, 
some of which are direct results of the methods of cultivation. Moreover, the greater part of our 
seed or fruit crops grow in a foreign or unfavourable climate, and are thus exposed to a number of 
unavoidable causes of disease.* 
The condition of a plant in cultivation may be regarded as identical with that of a plant in which 
disease has commenced. Almost all our cultivated plants have, by modification of the chemical pro¬ 
cesses of their vegetation, been caused to deviate from the normal type, and though we consider them 
improvements, because they are profitable to us, it is an improvement of the same sort as that where 
the enlarged liver is produced in geese for the Strasburg pies. This general morbid condition is 
heightened into specific predisposition to disease, when the conditions of cultivation are opposed too 
strongly or too suddenly to those of nature, as when natives of light or sandy soil, such as the Oat or 
Potato, are planted in heavy land; or when Wheat, Rye, or Barley are sown in land in the first year 
of its being manured ; or when the climate is very unlike the original one of the plant, as in the case 
of Maize in most parts of Europe. 
The outward forms of disease in plants are sufficiently known. The internal appearances are less 
understood, and, for their proper apprehension, require some knowledge of vegetable anatomy. The 
characters are essentially similar in all living vegetable cells; we have a wall or membrane composed 
of cellulose, devoid of nitrogen; this wall is lined by a semifluid layer of viscid mucilage (the pri¬ 
mordial utricle ) composed of a proteine-compound abounding in nitrogen; the cavity of the cell is filled 
up with watery juice containing little nitrogenous matter, but having all the other compounds, such as 
gum, sugar, vegetable acids, inorganic salts, &c., dissolved in it. The chemical force of the plant 
would appear to reside in the nitrogenous mucilaginous layer; all growth depends on this and it does not 
disappear, until the cell-wall (really the skeleton of the plant, like the horny or stony substance of the 
coral polypes) has become perfectly developed. 
Now, when diseased plants are examined in an early stage, whether it be smut in Wheat, rotting 
of succulent parts or of stems, or the Potato disease, the first morbid appearance is found in this nitro¬ 
genous mucilaginous layer, which becomes discoloured, coagulated, and granular; then it seems to 
penetrate into and affect the cellular wall of the cell. These appearances are so general that it may 
fairly be asserted that all internal diseases of plants commence in this way, in the nitrogenous in¬ 
ternal coating of the individual cells, in which their chemical force seems to be concentrated. 
Professor Schleiden adopts the view of Liebig, that all the variety of the vegetable world, so far as 
it depends upon chemical processes, arises exclusively from the varying qualitative and quantitative 
composition of the inorganic part of the soil; and that poor or luxuriant development, healthy or dis¬ 
eased condition, must be attributed to this. The experiments of Boussingault and Liebig show that 
the formation of the organic substances of the dextrine and the proteine series depends exclusivelv on 
the presence of certain inorganic substances. It is not yet decided whether the formation of the 
proteine (nitrogenous) subtances depends on the presence of phosphoric acid, and that of the dextrine 
series on the presence of alkaline salts without phosphoric acid, but the following facts render it 
probable:— 
* The unsatisfactory deduction from this, that •we must not hope to exterminate disease in our cultivated plants, even by 
improved knowledge, points to the natural mode of proceeding in such matters—to the principle of assurance —finding averages 
of the good and bad times, and making the former pay for the latter. 
