Page:Popular Science Monthly Volume 45.djvu/811

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Fig. 17. At this stage we should also expect the flowers to be solitary, arising each from the axil of a leaf very similar to the rest of the plant's foliage.

Competition in securing the benefits of insect visits, together with the possibilities of a more economical as well as more effective disposition of tissue-building material, would conspire to bring about through natural selection the following changes:

1. Those branches of the herb on which flowers appeared would be given up more and more fully to their function of flower production; their subtending leaves would be reduced in size, and through a shortening of the axis the flowers would be brought closer together, and thus their conspicuousness enhanced. At the same time, part of the material saved might go to form additional flowers in the cluster. With the assumption of the shrubby habit the floral branches (peduncle, rhachis, and pedicels) retaining their herbaceous nature, in consequence of their short-lived usefulness, would appear still less like the others. The formation of flower buds to last over the winter would favor the blossoming of the flowers more nearly together in the following

PSM V45 D811 Hypothetical stages of evolution in flowers.jpg
Fig. 17. Fig. 18.
Fig. 17.—Diagram showing number and arrangement of parts in the primitive berberidaceous flower. Bracts, three (heavy black); sepals, six (outlined); stamens, twelve, dehiscence longitudinal; carpels, sis, many-ovuled.
Fig. 18.—Diagram of flower (hypothetical) in a stage of evolution intermediate between the primitive form and the highest barberry type. Bracts and sepals as before; stamens, twelve, with valvular dehiscence and bearing nectar glands (heavy black); carpel, one, many-ovuled.

year. As the subtending leaves would now have lost almost the last vestige of their usefulness, we should expect their reduction to mere scales. The result of all this would be such a raceme as we find the barberry to possess (Fig. 2).

2. With the increase in the number of flowers in a cluster there would be less need for so many pistils in each flower. It might often happen that only a few of those in one flower would be fertilized, and in that case the store of food could be increased in the favored seeds, much to the advantage of the offspring produced. Pistils which ceased to have a use would gradually dis-