Birds and Nature, Vol. 08, No. 2, September 1900 / Illustrated by Color Photography

by Various

FLOWERS AND THEIR INVITED GUESTS.

It must be taken for granted in this paper that the reader has such knowledge of the parts of the flower as could be obtained from the paper on "A Typical Flower," printed in the June number.

When flowers first appeared it became necessary to secure the transfer of the pollen grains to the stigmas. This was necessary in order that the ovule might be developed into a seed containing a young plant or embryo. At first the currents of air were selected as the agents of this pollen transfer, and the flowers were adapted to what is known as wind-pollination. As the wind is an inanimate agent any transfer by it is largely a matter of chance. In order to increase the chances of successful pollination it was necessary for pollen to be developed in enormous quantities, so that it might fall like rain. In this way stigmas would be reached, but at the same time an enormous amount of pollen would be wasted. The evergreens are good illustrations of wind-pollinated plants, and their showers of pollen are very familiar to those who live near pine forests. When these showers come down in unaccustomed regions they are often spoken of as "showers of sulphur," and the local newspapers are full of accounts of the mysterious substance.

In wind-pollinated plants not only must the pollen be excessively abundant, but it must also be very light and dry. Sometimes the buoyancy is increased by the development of wings on the pollen grains, as in the case of pines. This habit of pollination is found not only among the evergreens, but also among many important families of the higher plants, as in the ordinary forest trees, the grasses, etc.

When the higher forms appeared, however, flowers of a different character gave evidence that a new type of pollination was being devised. Instead of the old wasteful method, insects were called in to act as agents of the transfer. By securing an animate agent there is a definiteness in the pollination and a saving in pollen production which is quite in contrast with the wind method. It must not be supposed that all flowers have learned to use insects with equal skill, for many of them may be said to be clumsy in their arrangements. On the other hand, certain families have reached a high degree of organization in this regard, and arrange for insect visits with a skill and completeness of organization which is astonishing.

In order to secure visits from insects, so that pollination may be effected, flowers have been compelled to do several things. In the first place, they must provide an attractive food. This has taken two prominent forms, namely, nectar and pollen. There are insects, such as butterflies, which are not only attracted by the nectar, but whose mouth parts have only been adapted for sucking up a liquid. There are other insects, however, like the bees, wasps, etc., which are able to take the more substantial pollen as food. Accordingly insects which visit flowers may be roughly divided into the two classes, nectar-feeders and pollen-feeders.

In the second place, the flower must notify the insect in some way that the food is present. This is done primarily by the odors which flowers give off. It must not be supposed that odors which are sensible to us are the only ones sensible to insects, for in general their sense of smell is far keener than ours. It is also probably true that the display of color, which is so conspicuously associated with flowers, is an attraction to insects, although this has become somewhat doubtful lately by the discovery that certain insects which were thought to be attracted by color have proved to be color blind. At present, however, we have no reason to suppose that color is not associated in some prominent way with the visits of insects.

It should be noticed, also, that two kinds of pollination are possible. The pollen may be transferred to the stigma of its own flower, or it may be carried to the stigma in some other flower, and this other flower may be some distance away. The former method may be called self-pollination, the latter cross-pollination. It seems evident that flowers in general have made every effort to secure cross-pollination. This would seem to imply that it is a better method for some reason, although we may not be able to explain why. Apparently, however, while flowers in general have tried to secure cross-pollination, they have not entirely abandoned the chances of self-pollination, so that if one should fail the other may be used. In this way it will be found that a great many plants have two kinds of flowers, the ordinary showy kind, and in addition to them inconspicuous flowers which are never seen except by those acquainted with their presence. For example, in the common violet, in addition to those flowers with which everyone is familiar, others are developed which are concealed by the cluster of leaves, which never open, but which are able to produce very well developed seeds.

With nectar and pollen provided as food, and with odor and color notifying the insects of their presence, it remains to be noted that the suitable insects are those which fly. A creeping insect is of no avail in the work of pollination, since the pollen will be rubbed from its body as it crawls from one flower to the next. How the flowers ward off the visits of creeping insects, which are attracted as well as the flying ones to the food provided, will be described in a subsequent paper.