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Updated: May 14, 2025
In these several cases, with the exception of that of the well-developed ray-florets, which are of service in making the flowers conspicuous to insects, natural selection cannot, as far as we can judge, have come into play, or only in a quite subordinate manner.
In my own cultures of the improved field-marigold I have observed it frequently. The number of the ray-florets may be considered as a direct response to nourishment, both when this is determined by external circumstances, and when it depends on the particular strength of the branch, which bears the head in question.
Seeds must be saved from less dense heads, and no way has been found of avoiding it. The ray-florets are sterile, even in the wild species, and when growing in somewhat large numbers on the disk, they conceal the fertile flowers from the visiting insects, and cause them also to be sterile. The same is the case with the best cultivated forms.
Many other instances could be given. One of the best authenticated cases is the dependency of partial fluctuation on the season and on the weather. Flowers decline when the season comes to an end, become smaller and less brightly colored. The number of ray-florets in the flower-heads is seen to decrease towards the fall.
The method is so simple and so striking, and so wholly independent of any mathematical development that it should be applied in all cases in which it is desired to ascertain the average value of any organ, and the measure of the attendant deviations. I cite an instance, secured by counting the ray-florets on the flower-heads of the corn-marigold or Chrysanthemum segetum.
With respect to the difference in the corolla of the central and exterior flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous in the fertilisation of plants of these two orders, is so far-fetched, as it may at first appear: and if it be advantageous, natural selection may have come into play.
In many cases they are sterile, having only an imperfect ovary. They are large and brightly colored and are generally designated as ray-florets. And the genus of the marigolds or Bidens is noted for containing both of these types. Here we have opportunity to observe positive and negative varieties of the same character.
These differences have been attributed by some authors to pressure, and the shape of the seeds in the ray-florets in some Compositae countenances this idea; but, in the case of the corolla of the Umbelliferae, it is by no means, as Dr. Hooker informs me, in species with the densest heads that the inner and outer flowers most frequently differ.
The Canada fleabane or Erigeron canadensis, the tansy or Tanacetum vulgare and some others may at times be seen with ray-florets, and according to Murr, they may sometimes be wanting in Aster Tripolium, Bellis perennis, some species of Anthemis, Arnica montana and in a number of other well-known rayed species.
Total lack of historical evidence concerning the origin of a variety has sometimes been considered as sufficient proof of a sudden origin. The best known instance is that of the renowned cactus-dahlia with its recurved instead of incurved ray-florets. It was introduced from Mexico into the Netherlands by Van den Berg of Jutphaas, under the following remarkable circumstances.
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