150 



KNOWLEDGE. 



[August 1, 1891. 



of flowers — !■.//., lU'rhfiis ruJiinris, the Barberry ; Suli-ia, the 

 Sage. 



The little discs to which the caudicles are attached 

 are sticky on the under surface. The viscid material 

 enables them to adhere to the proboscis or head of any 

 insect with which they come iu contact. The filament of 

 the stamen has not yet been seen by us — in fact, search for 

 it would be in vain, as it has united with the pistillate or 

 female portion of the flower to form the column. It is 

 evident, then, that only one stamen is jjresent in the 

 flower. A vertical section through the rostellum of 

 another flower which has not been robbed of its pollinia 

 reveals the source of the \ascid material of the disc, and 

 also of the disss themselves, for the rostellum is a thin 

 membrane (Fig. I., 1 »(.) whose interior is lined with a 

 graniflar fluid (Fig. I., 1 ;/.) that surrounds two balls of 

 viscid matter, only one of which is seen in Fig. I., 1. 

 Each ball is attached to a circular piece of thin membrane, 

 thinner than the rest of the membrane of the rostellum, 

 but confluent with it. As is endent from the figui-e, the 

 discs are develo^jed towards the back of the flower and the 

 pollinium assumes a curved position in its case. 



Situated beneath the rostellum are two protruding 

 sticky patches almost imited to one another. They are 

 the stigmatic surfaces (Fig. I., 1 ). The flower is supported 

 on a thick twisted stalk. Fig. III., 4, is a section of the 

 similar stalk of Habeuaria, the Orchid we are next to 

 consider. It is Uned with three longitudinal rows of 

 ovules, and so it is the ovary of the flower. It is important 

 to note that it is twisted. The style (that portion of the 

 pistil that is generally elongated and bears the stigma on 

 its summit) is here very short, forming part of the column, 

 as already noted. 



A wonderful mechanism comes into play in connection 

 with the removal of the viscid discs. Whenever an insect 

 touches the rostellum, the delicate membrane ruptures 

 before and behind, so as to leave the discs free. At the 

 same instant it springs downwards and exposes the balls 

 of sticky matter, which then come in contact with the 

 animal's body. The manner in which fertilization is 

 accomplished is as follows ; — An insect alights on the 

 labellum, and in endeavouring to push its proboscis into 

 the nectary to obtain honey, it infallibly knocks its head 

 against the irritable rostellum. The mechanism already 

 described then comes into action, and the insect leaves 

 the flower with one or both j)ollinia firmly affixed to its 

 head. It may repeat the process on other flowers of the 

 same spike, so that when it flies ofl' from it many pollinia 

 may be attached to its body. Darwin, to whom we are 

 chiefly indebted for our knowledge of the processes of 

 Orchid pollination, says that he foimd ten to sixteen 

 pollinia attached to the bodies of live bees that had been 

 visiting an ally, Orchis morio. 



Bees only spend three or four seconds on each flower ; 

 hence it is evident that the sticky material of the discs 

 cannot take long to harden, and this is so. It is also 

 quite apparent that unless the polhnia changed their 

 position on the insect's head that insertion of the proboscis 

 into the nectary of another flower would simply lodge the 

 pollinia relatively in their old position. To prevent this, 

 a marvellous contrivance is exidbited. The small viscid 

 disc has such power of contraction that it causes the 

 pollinium to move in a forward direction through an angle 

 of 90°, thus bringing it into a position that ensures its 

 impinging on the stigma of another flower visited by the 

 bee. It must be imderstood that the discs are firmly 

 fastened to the insect's head. Were it not so, in moving, 

 they might assume a lateral du-ection, which would prevent 

 them performing the act of poUiuation. The cement sets 



hard, according to Miiller, in from three to five seconds 

 when exposed to the air, and this is the time occupied by 

 the bee in visiting a single flower. About forty seconds, as 

 a rule, elapse before the pollinia bend forward. A bee, as a 

 rule, spends twenty-five seconds on a single spike ; thus, 

 fertilization -vdih pollen from the same inflorescence is 

 obviated. The time occupied in flying to another spike 

 gives the pollinia the opportunity of assuming the position 

 requisite for the performance of the act of poUmatiou. 



It frequently happens that only one pollinium is removed 

 at a time ; hence, imless some special contrivance were pro- 

 vided, the other pollmium woifld be useless owing to the 

 exposure of its cement to the air by the lowering of the 

 membrane of the rostellum. This is obviated by the power 

 inherent iu this membrane of springing back to its place 

 immediately a pollinium has been removed. 



Miiller records an interesting study he made of the cross- 

 fertilization of this Orchid. " On May 6th, 1869, I and 

 my son Hermann at length succeeded in observing humble- 

 bees fertilizing the flowers of Orchis upon Stromberg HUl. 

 As we lay upon the turf, which was overgrown with ihchis 

 mustula, we saw a humble-bee (apparently IJuml/Ms teinKtris) 

 alight, close beside us, on the base of a spike of that plant. 

 It thrust its head into a flower, and drew it out, after about 

 four seconds, with the pollinia attached to it. It repeated 

 the same operation on two more flowers. After withdi-aw- 

 ing its head fi'om the third, it paused, and tried without 

 success to free itself from the pollinia, which were cemented 

 firmly to the front of the head. Clirabhng a little fiu-ther 

 up the spike, it thrust its head into a fourth flower. . . . 

 Of ninetj^-seven humble-bees which we caught on that 

 day on Stromberg Hill, thirty-two bore pollen-masses of 

 Orchids." 



Although one repeatedly examines the nectaries of Onhis 

 vuixcula, one always finds them destitute of nectar. This 

 was a puzzling circumstance to Darwin. He could not 

 account for the continued nsits of insects to these flowers 

 unless by the fact that they themselves benefited by it. 

 In fact, so strongly convinced was Sprengel that no nectar 

 was produced under any circumstance, that he called them 

 sham-nectaries. Darwin, however, could not believe that 

 insects would allow themselves to be so systematically 

 duped. But not a drop of nectar was revealed even by 

 microscopic examination. At last he found a clue. The 

 spur of the labellum has a comparatively thick wall, and 

 its inner membrane is dehcate ; moreover, the juicy natm-e 

 of its contents is very evident. In Habenaria there is a 

 copious supply of nectar, and the wall of the spur is 

 thin. Darwin observed that bees which visited the ilowers 

 of On-hix morio "remained for some time with their 

 proboscides inserted into the diy nectaries, and (he) 

 distinctly saw this organ in constant movement. (He) 

 observed the same fact with Empis in the case of Orchis 

 mueulata, and on afterwards opening several of the 

 nectaries (he) occasionally detected minute brown specks, 

 due as (he) believed to the punctures made some time 

 before by these flies." This then is the solution of the 

 problem — the insect bites with its mandibles the fine 

 inner membrane of the wall of the spur, and sucks out 

 with its proboscis the fluid contained therein. 



It will be remembered that the pollinia cannot be 

 afiSxed to the head of the insect visitor imless the cement 

 of the disc has hardened. This requires about three or 

 four seconds, the time spent by the insect in piercing the 

 waU of the nectary. We see then how beautifully, even 

 to the minutest detail, this flower has been modified in 

 order to ensure its cross-fertihzation. 



For its intrinsic interest and as afl'ording a contrast 

 to the preceding, we will examine the structm-e and mode 



