In the September number we considered flowers and their invited guests,
that is the insects useful in carrying pollen. The very things which
attract useful insects to flowers are attractive also to useless
insects. For example, nectar in a flower seems just as desirable to
an ant as to a butterfly, but the ant is a creeping insect and would
be likely to lose the pollen in passing from one plant to another. If
useless insects found free access to flowers and carried off their
food supplies, the useful insects would soon stop visiting them. It
is of great advantage to flowers, therefore, to have some means of
warding off the creeping insects. It must not be understood that all
plants are equally successful in this matter, or that any plant is
always successful, but there are certain things which seem to hinder or
discourage the approach of creeping insects to flowers.
Perhaps ants may be taken as the best illustration of the insects whose
visits are discouraged by flowers. They are very much attracted to
the food supplies in the flower, especially the nectar, and are among
the most intelligent of insects, often overcoming the most serious
obstacles. They will be considered in this paper, therefore, as the
insects which are seeking the nectar and pollen of flowers without
invitation. A charming little book upon this subject has been written
by Kerner, and translated into English, under the title which appears
at the head of this paper. It is in this book that the chief obstacles
to such unwelcome guests as ants are clearly stated.
Hairs.--One of the most common obstacles to ants is a barrier of hair.
For some reason, ants dislike to cross such a barrier. Travellers in
tropical countries, where ants abound, tell us that a hair rope laid
around a tent is a very effective barrier against the invasion of
armies of ants. Hair is very commonly found upon plants, and it may be
noticed that it is apt to increase in amount and prominence towards or
within the flower cluster. Sometimes the flowers themselves are hairy
outside, and in the case of the trailing arbutus, whose flowers close
to the ground are in special danger from creeping insects, the flowers
are filled with a fluffy mass of hairs. In our illustrations, the wild
columbine, the Oswego tea, the sunflower, and the ox-eye daisy are
all hairy plants, and difficult for ants to climb. In the September
number are illustrations of the mallow, the lady's slipper, and the New
England aster, all of which are hairy and discouraging to ants.
An interesting fact in connection with the wild columbine may be
noted. The nectar is deposited in the knob-like bottom of the long
tubular spurs, and the entrance is so carefully guarded that only a
long and slender proboscis, like that of a moth or a butterfly can
reach the nectar. The bumblebees, however, have learned this fact,
and bite through the tips of the spurs and steal the nectar. As a
consequence, the wild columbine is said to be little visited by the
proboscis-bearing insects, and its pollination is seriously interfered
with.
Sticky excretions.--Some plants have the power of excreting upon their
surface a sticky substance like mucilage. This mucilage may be produced
by hairs, which are then called "glandular hairs," or it may appear
directly on the surface of the plant. When ants or other insects try
to cross such a barrier they are not merely stopped but caught. Upon
"glandular" plants it is very common to see small insects stuck fast,
and it is more than probable that the nourishing material of their
bodies is digested and absorbed by the plant. In this way the plant not
merely stops the insect, but catches and devours it.
A very common illustration of such a plant is the "catchfly," whose
name suggests its power. The joints of its stem are long, and near the
upper end of each joint is a band of mucilage. This series of sticky
bands forms a very effective barrier to any insect trying to crawl up
the stem.
Isolation.--In some cases plants or their flowers are isolated from
creeping insects by water, which forms a most efficient barrier. This
has been demonstrated by housekeepers, who in the days of "safes" were
accustomed to set the legs of the safe in cans of water to ward off
the invasion of ants. Of course, plants standing in the water are well
isolated, and usually show no further device for warding off creeping
insects. There is an interesting fact connected with one of our water
smartweeds, which has to do with our subject. Ordinarily it stands in
shallow water, and is perfectly smooth; but when occasionally the water
dries up the plant becomes hairy. That this has anything to do with
the danger from creeping insects is unlikely, but the hairy covering
certainly appears at a very opportune time.
The teasel was once extensively cultivated as a fuller's plant, and one
or two species of it have become common as wild plants, their dense
and prickly flowering spikes looking like swabs for cleaning lamp
chimneys. The plant is tall and coarse, and is peculiar in that its
large opposite leaves unite by their bases about the stem to form a
cup. In this way a series of cups is developed on the stem, and in each
cup there is water. When a creeping insect crawls over the edge of the
cup he sees the stem rising from a pool of water which must be crossed.
As there is a series of such pools it is very unlikely that any such
insect reaches the showy cluster of flowers.
The so-called "travelers' tree" of the tropics is a teasel upon
a larger scale. The enormous flower cluster is at the top of the
plant, and between it and the ground is a series of very large
water-containing cups formed by the leaves. The popular name has been
given by travelers who have been represented as reaching a cup with a
spear and piercing it, thus obtaining a supply of water. The story is
very doubtful, and the water, usually full of the macerating bodies of
insects, is still more doubtful.
Latex.--By this term is meant the milky juice which some plants
possess. When such a plant is punctured or torn the latex flows out,
and as soon as it is exposed to the air it becomes more and more sticky
until it hardens. It is from the latex of certain trees that India
rubber is obtained, but it may be observed in many plants, notably the
milkweeds, which have received their popular name on account of it.
The milkweed may be used to illustrate how latex may be of service
in warding off creeping insects. In many cases the plant is entirely
smooth, and the stems of the flower cluster are even slippery. When
an ant reaches these slippery surfaces it clutches for a hold and its
sharp claws pierce the tender skin of the plant. Immediately a drop of
latex oozes out and becomes sticky, and when the ant seeks to lift its
feet there is resistance, and in the struggle the claws clutch deeper,
more latex oozes out and becomes more and more sticky, until finally
the insect is stuck fast. The flower clusters of certain milkweeds are
often found plentifully covered with small captured insects.
Protective shapes.--Many flowers secrete their nectar so that a
creeping insect cannot reach it, but the suitable insect can.
Illustrations are numerous, but the following will suffice.
The wild columbine, represented in one of our illustrations, secretes
its nectar at the bottom of long tubular spurs, which can be traversed
by slender probosces, but are impassable to creeping insects. Spurs are
developed in many flowers, notably the orchids, and they are always
associated with nectar secretion and the visits of proboscis-bearing
insects.
In the Pentstemon, a plant whose flowers have two lips, as in the
Oswego Tea in our illustration, but not so prominent, the nectar is
secreted in a little pit. Across the mouth of this pit one of the
stamens, modified for this purpose, is placed like a drop-bar, leaving
but a thin crevice leading to the nectar pit. Through this crevice a
thread-like proboscis may be thrust, but a creeping insect cannot pass.
In the snapdragon the two lips of the flower are tightly closed,
the lower one decidedly projecting. Any small insect reaching this
lower lip as a natural landing place finds no entrance. When the
bumble-bee alights upon the lower lip, however, his weight depresses
it and he forces his way in, and in passing to another flower effects
pollination. It is interesting to note that after this first and
important visit the lips remain open and other insects pass in freely,
"being invited," as some one has said, "to eat at the second table."
In most of the orchids there is a very complete adaptation of the
flower to its insect, by which almost every insect excepting one
special kind obtains nothing for its visit. The nectar is usually in
the end of a long spur, and to obtain it the head of the insect must
just fit between two sticky buttons to which the pollen-masses are
attached. The length of the spur is nicely adjusted to the length of
the proboscis of the visiting insect, but his head must also be of a
certain breadth. If an insect visits the flower with a proboscis too
short or too long, or a head too broad or too narrow, its visit is
unavailing. The danger of such narrow specialization is apparent in
the case of the orchids, for each plant is so dependent upon a special
insect that the disappearance of the latter seriously endangers the
continued existence of the former.
Protective closure.--It has long been noticed that certain flowers open
only in the evening, the evening primrose being a conspicuous example.
These flowers are adapted to the visits of the night-fliers, the moths,
and about clusters of evening primroses numerous large hawk-moths may
be seen after sunset. During the day the flowers are closed and safe
from the visits of any insect, but by opening in the evening they are
not only ready for the visits of the night-fliers, but they avoid the
visits of most creeping insects, notably the ants, who are not abroad
after "the dew falls."
Protection against grazing animals.--Although we are considering the
ways by which creeping insects are checked in their efforts to visit
flowers, it seems pertinent to mention the more universal danger which
comes from grazing animals. If flowers were as attractive to grazing
animals as they are to insects they would be in danger of wholesale
destruction. It can be observed, however, that these animals as a rule
avoid the flowers of a plant, although they may strip off its leaves.
It is believed that this avoidance is due to the fact that in or about
the flower cluster there are usually secreted bitter, sour, or nauseous
substances, which grazing animals have learned to avoid. It should
not be imagined that these substances are there for that purpose, but
being there the result is that the flowers are avoided. It is unknown
how generally true this is, and the effectiveness of this method of
protection may have been exaggerated. Those who can observe cattle,
however, are in a position to test them with the flowers of the various
plants they are known to eat, and determine how far they avoid them.
In conclusion, it may be of interest to call attention to the great
complexity of relations existing among plants and animals by repeating
Darwin's famous illustration known as "Cats and Clover." In a certain
district in England he observed that the clover was pollinated by
the bumblebees, which had their nests in the fields. It followed,
therefore, that the more the bumblebees, the more the clover. He also
observed that the field mice preyed upon the young broods of the
bumblebees, and, therefore, the more the field mice, the fewer the
bumblebees and the less the clover. When cats were plenty and preyed
upon the field mice it followed that the more the cats, the fewer the
mice, the more the bumblebees, and the more the clover. Therefore, the
crop of clover depended upon the presence of cats in the neighborhood.
John Merle Coulter.