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Bye-and-bye a humble-bee wanders along, quickly finding that another has drained the blossoms of their sweets. He passes on undismayed; there are more flowers. Over by the wire fence the tick-trefoil, desmodium, is in its glory. Its lower petal stands out like a doorstep, and on it the humble-bee alights. Two little yellow spots, bordered with deep red, show him where lies the nectar.
The desmodium and genesta celebrate their hospitality with a joke, as it were, letting their threshold fall beneath the feet of the caller, and startling him with an explosion and a cloud of yellow powder, suggesting the day pyrotechnics of the Chinese. The prickly-pear cactus encloses its buzzing visitor in a golden bower, from which he must emerge at the roof as dusty as a miller.
Fortunately the direct proof of this assertion can be given, and in a case which is narrowly related, and quite parallel to that of the Desmodium, since it affects a plant of the same family. It is the case of the monophyllous variety of the bastard-acacia or Robinia Pseud-Acacia. In a previous lecture we have seen that it originated suddenly in a French nursery in the year 1855.
But the most curious instance is that of the telegraph-plant, or Desmodium gyrans, each complete leaf of which consists of a large terminal leaflet and two little lateral ones. These latter keep up, night and day, an irregular jerking movement, which has been compared to the movements of a semaphore.
Other leaves produce one or two small leaflets at the base of the large terminal one, and by this contrivance are seen to be very similar to those of the Desmodium, repeating its chief characters nearly exactly, and only differing somewhat in the relative size of the various parts. Lastly real intermediates are seen between the monophyllous and the pinnate types.
Desmodium is a papilionaceous plant and closely allied to the genus Hedysarum, which has pinnate leaves with numerous pairs of leaflets. Its place in the system leaves no doubt concerning its origin from pinnate-leaved ancestors. At the time of its origination its leaves must have become reduced as to the number of the blades, while the size of the terminal leaflet was correspondingly increased.
As far as I have been able to ascertain, these are produced on weak twigs under unfavorable conditions; the size of the terminal leaflet decreases and the number of the lateral blades increases, showing thereby the presence of the original pinnate type in a latent condition. The sudden origin of this "one-leaved" acacia in a nursery may be taken as a prototype of the ancient origin of Desmodium.
The desmodium, the bidens, the agrimony and the cocklebur, which stick to your clothes even as late as February, are only using you as a Moses to lead their children to their promised land.
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