the formation of vegetable mould-第12章
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having had their bases gnawed had been pushed out of the burrows
and were now lying on the ground。 From these several facts we may
conclude that worms draw in some petioles of the ash by the base to
serve as food； and others by the tip to plug up the mouths of their
burrows in the most efficient manner。
The petioles of Robinia pseudo…acacia vary from 4 or 5 to nearly 12
inches in length； they are thick close to the base before the
softer parts have rotted off； and taper much towards the upper end。
They are so flexible that I have seen some few doubled up and thus
drawn into the burrows of worms。 Unfortunately these petioles were
not examined until February； by which time the softer parts had
completely rotted off； so that it was impossible to ascertain
whether worms had gnawed the bases； though this is in itself
probable。 Out of 121 petioles extracted from burrows early in
February； 68 were imbedded by the base； and 53 by the apex。 On
February 5 all the petioles which had been drawn into the burrows
beneath a Robinia； were pulled up； and after an interval of eleven
days； 35 petioles had been again dragged in； 19 by the base； and 16
by the apex。 Taking these two lots together； 56 per cent。 were
drawn in by the base； and 44 per cent。 by the apex。 As all the
softer parts had long ago rotted off； we may feel sure； especially
in the latter case； that none had been drawn in as food。 At this
season； therefore； worms drag these petioles into their burrows
indifferently by either end； a slight preference being given to the
base。 This latter fact may be accounted for by the difficulty of
plugging up a burrow with objects so extremely thin as are the
upper ends。 In support of this view； it may be stated that out of
the 16 petioles which had been drawn in by their upper ends； the
more attenuated terminal portion of 7 had been previously broken
off by some accident。
Triangles of paper。Elongated triangles were cut out of moderately
stiff writing…paper； which was rubbed with raw fat on both sides；
so as to prevent their becoming excessively limp when exposed at
night to rain and dew。 The sides of all the triangles were three
inches in length； with the bases of 120 one inch； and of the other
183 half an inch in length。 These latter triangles were very
narrow or much acuminated。 ｛32｝ As a check on the observations
presently to be given； similar triangles in a damp state were
seized by a very narrow pair of pincers at different points and at
all inclinations with reference to the margins； and were then drawn
into a short tube of the diameter of a worm…burrow。 If seized by
the apex； the triangle was drawn straight into the tube； with its
margins infolded； if seized at some little distance from the apex；
for instance at half an inch； this much was doubled back within the
tube。 So it was with the base and basal angles； though in this
case the triangles offered； as might have been expected； much more
resistance to being drawn in。 If seized near the middle the
triangle was doubled up； with the apex and base left sticking out
of the tube。 As the sides of the triangles were three inches in
length； the result of their being drawn into a tube or into a
burrow in different ways； may be conveniently divided into three
groups： those drawn in by the apex or within an inch of it； those
drawn in by the base or within an inch of it； and those drawn in by
any point in the middle inch。
In order to see how the triangles would be seized by worms； some in
a damp state were given to worms kept in confinement。 They were
seized in three different manners in the case of both the narrow
and broad triangles： viz。； by the margin； by one of the three
angles； which was often completely engulfed in their mouths； and
lastly； by suction applied to any part of the flat surface。 If
lines parallel to the base and an inch apart； are drawn across a
triangle with the sides three inches in length； it will be divided
into three parts of equal length。 Now if worms seized
indifferently by chance any part； they would assuredly seize on the
basal part or division far oftener than on either of the two other
divisions。 For the area of the basal to the apical part is as 5 to
1； so that the chance of the former being drawn into a burrow by
suction； will be as 5 to 1； compared with the apical part。 The
base offers two angles and the apex only one； so that the former
would have twice as good a chance （independently of the size of the
angles） of being engulfed in a worm's mouth； as would the apex。 It
should； however； be stated that the apical angle is not often
seized by worms； the margin at a little distance on either side
being preferred。 I judge of this from having found in 40 out of 46
cases in which triangles had been drawn into burrows by their
apical ends； that the tip had been doubled back within the burrow
for a length of between 1/20 of an inch and 1 inch。 Lastly； the
proportion between the margins of the basal and apical parts is as
3 to 2 for the broad； and 2。5 to 2 for the narrow triangles。 From
these several considerations it might certainly have been expected；
supposing that worms seized hold of the triangles by chance； that a
considerably larger proportion would have been dragged into the
burrows by the basal than by the apical part； but we shall
immediately see how different was the result。
Triangles of the above specified sizes were scattered on the ground
in many places and on many successive nights near worm…burrows；
from which the leaves； petioles； twigs； &c。； with which they had
been plugged； were removed。 Altogether 303 triangles were drawn by
worms into their burrows： 12 others were drawn in by both ends；
but as it was impossible to judge by which end they had been first
seized； these are excluded。 Of the 303； 62 per cent。 had been
drawn in by the apex （using this term for all drawn in by the
apical part； one inch in length）； 15 per cent。 by the middle； and
23 per cent。 by the basal part。 If they had been drawn
indifferently by any point； the proportion for the apical； middle
and basal parts would have been 33。3 per cent。 for each； but； as we
have just seen； it might have been expected that a much larger
proportion would have been drawn in by the basal than by any other
part。 As the case stands； nearly three times as many were drawn in
by the apex as by the base。 If we consider the broad triangles by
themselves； 59 per cent。 were drawn in by the apex； 25 per cent。 by
the middle； and 16 per cent。 by the base。 Of the narrow triangles；
65 per cent。 were drawn in by the apex； 14 per cent； by the middle；
and 21 per cent。 by the base； so that here those drawn in by the
apex were more than 3 times as many as those drawn in by the base。
We may therefore conclude that the manner in which the triangles
are drawn into the burrows is not a matter of chance。
In eight cases； two triangles had been drawn into the same burrow；
and in seven of these cases； one had been drawn in by the apex and
the other by the base。 This again indicates that the result is not
determined by chance。 Worms appear sometimes to revolve in the act
of drawing in the triangles； for five out of the whole lot had been
wound into an irregular spire round the inside of the burrow。
Worms kept in a warm room drew 63 triangles into their burrows；
but； as in the case of the pine…leaves； they worked in a rather
careless manner； for only 44 per cent。 were drawn in by the apex；
22 per cent。 by the middle； and 33 per cent。 by the base。 In five
cases； two triangles were drawn into the same burrow。
It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex； not from the
worms having selected this end as the most convenient for the
purpose； but from having first tried in other ways and failed。
This notion was countenanced by the manner in which worms in
confinement were seen to drag about and drop the triangles； but
then they were working carelessly。 I did not at first perceive the
