Page:The New International Encyclopædia 1st ed. v. 07.djvu/858

* FLOWERS AND INSECTS. 762 FLOWERS AND INSECTS. The immediate cause of the shape of the wing- petals of many papilionaceous flowers is at- tributed to the "weight of the insect in front, the loeal irritations behind, due to the thrust of the insect's head in probing for nectar, coupled DAVEItNOIA ADHATODOIDES. a. The Hi iwer, an example of an irregulars > roll a adapted to the needs of its insect visitors; /'. appearance when en- tered by a bee. "The weight of the bee must be very great, and the curious shape of the lip with its lateral ridges is evidently not only an excellent landing-place, but is so constructed as to bear that weight. Moreover, the i u o walls slope off and are gripped by the legs of the bee so that it evidently can serim- an excellent purchase, and thus rifle the flower of its treasure at its ease." (ffensiow.} with the absence of all strains upon the sides." Good examples of the occurrence of great thick- ening of the tissue just where the strain will be most felt are to be seen in the slipper-shaped flowers Cypripedium, Calceolaria, etc. If the reader will turn to the article Kineto-* genesis, he will see how exactly parallel are the cases cited with those mentioned for plants by llenslow. In part he says: "In alluding to the above instances of levers and mechanical powers in plants, one mentally recalls how abun- dant they are in the distribution of the bones and muscles in vertebrates. I cannot help thinking, therefore, that the old view was fundamentally correei ; that such have been gradually brought into existence by the efforts to meet the --train put upon them. If this be true, then one and the same law has prevailed in the evolution of organs in both the animal and vegetable kingdoms." The accompanying figure ex- plains how the forms of the calyx and corolla are adjusted td bear the weight of the insect. The bee, he says, alights on the lip and then partly crawls into the expanded mouth of the C0- i.amiiu, rolla. -'> that its weight now A finwer of Lnmi- lies in the direction of w. The am album, show- f u i crU m will he at f, ami the ing distribution of ni.ru, t lnder inseet resultant of these is m the visitation: n%diree- opposite direction to r. This is Hon of pressure w here the -Main will be felt ; From i be insect s k ,,,.•,-• , ,, . weight; /'. fulcrum; SO that it is just at this point r, resultant, oppo- where ile- backward curvature dte to the direction takeg , , | : , , ., . „|, i( .|, g j ves strength to th< rolla-tube. This latter reatly supported by die tube of the calyx. which, tted, has a curiously thickened cylin- der within tin- mesophyl." Other structures, as projecting hairs, arc so situated as to form ob tractions to the entry of small inserts which would be unable lo pollinate the flower. In th- gentian there are tooth like processes at the entrance of the corolla ; in the Indian pipe and a Daphne a large circular stig- ma nearly blocks up the tube. Insects have been repeatedly observed to lick tie- various parts of flowers, ami thus maintain an intermittent irri- tation and consequent formation of hairs and other products. Miiller has often watched a fly (Rhingia rostrata) licking the stamina] hairs of a Verbascum. anil in many cases the hairs on the filaments oiler a foothold to the insects while visiting the flowers, as in the mullein; such hairs, Henslow claims, "being the actual result of the insects clutching the filaments or rubbing them with their claws." The chief attraction of flowers to insects i- of course their honey-glands or nectaries, ami these are thought by Henslow to have originated from the visits of insects which, at first attracted to the juicy tissues of flowers, "by perpetually with- drawing fluids have thereby kept up a flow of the secretion which has become hereditary, while the irritated spot has developed into a glandular secreting organ." These spots occur at different places in different flowers, "wherever the pre- vailing insect found it convenient to search." On the other hand, nectaries disappear, when the whole flower degenerates and becomes regularly self-fertilizing or else anemophilous, so that insects do not visit them. Henslow even suggests that the insectivorous pitchers of Nepenthes may have been due to the external irritation caused by insects, for Sir J. D. Hooker had already shown that they originate from water-glands. The continuous flow of nectar has its analog^' in the daily regular, though intermit- tent, mechanical irritation, and its inherited effects, which keep up the secretion of milk in the goat and cow. Ants play their part in bring- ing about changes in what are called, for this reason, 'ant plants.' The hypertrophied stipules or thorns of Acacia, and the stems of Myrme- codium, as shown by Beecari. are due to the irritation set up by these insects, which bring about a. hypertrophy of the cellular tissue. A small swelling appears on the tigellum of Myrme- codium, serving the purpose of a reservoir of water, which only grows larger through the agency of ants. Henslow also attributes the large honey-pits at the base of the leafstalks on Acacia sphferocephala, as well as the terminal 'fruit bodies' occurring on the tips of the leaf- lets, to the same cause — viz. the mechanical irri- 1 .1 1 ion of the ants. Colobs. The colors of flowers are primarily due to nutrition. The parti-colored spots and streaks leading down into the bottoms of flowers, called 'guides' ami 'pathfinders,' invariably lead to the nectaries, and these effects seem directly due I" tin- visits of insects. "The guide-." -ays Henslow, "like obstructing tangles of hair ami nectaries, are always exactly where the irritation would be set up. anil I take them to be one result i,f n more localized flow of nutriment to the posi- tion- in question. Instead, therefore of a flower having tii-t painted a petal with a golden streak to invite the insect, ami to show it the right way of entering, the flrsl insect visitors themselves induced the flower to do it. ami so benefited all future comers." The facts and theories suggested by the colors of flowers are further borne oul by the geological history of fossil How, ring pi. mis and of those orders of insects containing