i 
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|) OX VAEIEGATION IN PLANTS. 
especially those of the sea, are animated by vast numbers of minute microscopic animals, called infu- 
sorial animals, each class of which is so organised as to live in its own special clement only. Near 
the mouth of rivers, where salt and fresh waters mingle, myriads die daily, mix with the mud of 
the river, and are deposited with it along the banks, contributing much to the fertility of the alluvial 
deposits in these localities. The skeletons or envelopes of these infusorial animals, consisting chiefly of 
silica, after the death of the animalcules, appear as a whitish slime, which, notwithstanding the fact 
that many billions of these little creatures are required to fill the space of one cubic inch, nevertheless 
increase considerably the fine mud which the river deposits along its course near the sea-shore. Thus 
the mud which subsides on the shores of Northern Germany, and in the harbours along the coast, 
consists for the greater part of such organic remains. Higher up in the rivers their number decreases, 
and in places beyond the reach of the tide none of them are found. In the Elbe these animalcules 
have been observed as far as eighty miles above its mouth. In all tidal rivers, for instance the Rhine, 
the Thames, the Wash, the Forth, and the Humber, the mechanical debris brought down by these 
rivers, on being deposited, are intimately mixed with these more fertilising animal remains, and thus 
alluvial soils of the utmost degree of fertility are formed. 
Although abundant proof has been advanced to convince the reader that a close general relation 
exists between the soil and the rock on which it rests, the supposition that this intimate connexion 
existed in every locality would lead to serious mistakes. In many places, indeed, the surface soils 
are totally different from those which would be formed by the degradation and decomposition of the 
subjacent rocks. Soils derived wholly from the rock immediately below them are confined to par- 
ticular situations, such as steep escarpments and high level grounds, whereas the class of soils which 
do not partake of the general character of the subjacent rocks are by far the most numerous. The 
above observations on the formation of soils afford a ready explanation of this apparent geological 
anomaly ; for when the chemical causes have prepared the ingredients of a rock for constituting a 
soil, other causes, — for instance, the mechanical influence of running water, — may interfere to trans- 
port them to a distant bed of very different composition. 
Such are some of the more obvious and important causes which contribute to the change and 
formation of soils, as we observe them in action around us at the present day ; and it is fail- to assume 
that in past time they have been in operation, in conjunction with other mightier influences, in pro- 
ducing the present aspect of the earth's surface. 
ON VAEIEGATION IN PLANTS* 
By Dr. MOREEN, Professor of Botany in the University of Liege. 
THE TKUE CAUSE OF VAEIEGATION. 
WHEN we see a form of variegation strictly confined to the nerves of the leaf; when we see the dis- 
coloured tissue abruptly limited to the median nerve, we are led to infer that here there exists 
an action which depends on the fibrous system. AVc arc confirmed in this opinion when we observe 
those beautifully reticulated leaves, in which the whole fibrous network is white or yellow, while the 
intervening spaces (intervenium) remain green. But when the number of examples brought under 
consideration becomes multiplied, the frequent examples of haws which are marginatc, or bordered, 
or zoned, or discoidal, or fasciated, or variegated at the ends, present facts which could never be made 
to accord with this hypothesis. In the leaves which arc fasciated, and those which arc variegated at 
the end, the nerves, like the intervenium, are distinctly cut by a discoloured part, and the transition is 
abrupt. It would, therefore, seem impossible to admit that this phenomenon of variegation is caused 
by any excess of air forced into the leaf by the pneumatophorous vessels, when the root itself would, 
in too poor a soil, take up more air than sap. One might think so, as respects the leaves described 
under the names of maculo-varicgatc, reticulate, vittate, marbled, and half variegated; but the mar- 
ginate leaves alone would overturn this theory. 
The nature of our studies has naturally led us to submit the whitened tissues to a microscopic 
inspection, for it has appeared to us essential to examine first the differences of tissue, which might 
exist between the parts which are green and those which are variegated, ami then Beek for the cause 
of this strange phenomenon. We cannot lure review all the features of the anatomy examined; 
besides the constant similarity which they presented leads us to think that the cause is the same under 
all circumstances. AVc therefore confine our remarks to C'ornus mascula, Euonymns japouicus, and R 
Syringa vulgaris. S 
• From Dodonaa, <m Secueii cPotservationa </'■ Botanique. Brussels. 
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