Birds and Nature Vol. 08, No. 3, October 1900 / Illustrated by Color Photography

by Various

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.