FIELD CLUB RAMBLES.

Otago Witness, Issue 3247, 7 June 1916, Page 62

FIELD CLUB RAMBLES.

By G. M. Thomson, F.L.S. Continuing my remarks on the fertilisation of the flowers of Pterostylis from where I Jef't oh last week I quote further from Cheeseman as follows : "Now, let us suppose that an insect were to enter a freshly-opened iiower. The only entrance is between the lips of the lateral sepals, and here tho apex of the lip is placed exactly where our visitor would probably alight.' At first the weight of the insect would most likely counteract the natural tendency of the lip to move inwards, but as the* insect crawls further into the flower this would have less effect, until at length the irritability of the lip would enable it to overcome the resistance offered and to spring" back to the column. If no capture is made the lip soon regains its former position, but if the insect is imprisoned it remains firmly oppressed to the column, while its prey continues to move about. For the prisoner there is new only one mode of escape. This is by crawling up the column, passing over the stigma and viscid rostellum, and finally emerging from between the appendages of the column, directly in front of the anther. This passage is, however, so narrow and confined that it would not be possible for an insect to pass through without brushing against the rostellum and detaching portions of its viscid surface. If the insect were now to touch the anther —and it is difficult to see how it can escape without doing so—one or more of the pollen-masses, lying loose in their cells, would become glued to the viscid matter on the insect's back and consequently be withdrawn from the flower. To understand the mode of fertilisation we have only to suppose that the insect, with the pollinia attached to it, visits another flower, and is again' imprisoned, when it is evident that in its efforts to escape it would pass aver and in front of the stigma, which is sufficiently ' adhesive when touched to draw off a portion of a pollenmass, or even a whole one, from the back of the insect. In order to satisfy myself that the lip really plays the important part I had supposed, I selected 12 flowers, which were just expanding, and removed that organ from the whole of them. After a week or two, when they had closed and commenced to wither, 1 gathered them and examined their stigmas and pollinia. Not one flower was fertilised, and not a single pollen-mass had been removed. "On several occasions I have artificially enclosed small insects in the flower. Most escaped by crawling up the column and passing between the appendages, and some, but not all, carried pollen-masses with them. It can hardly bo expected, however, that insects selected at random would remove the with the same ease and certainty as the species to whose requirements the flower has no doubt been profoundly modified by natural selection, acting during long periods of time." [I have frequently repeated Mr Checseman's but as I have never caught an insect visiting a flower of Pterostylis of its own accord I do not know, any more than he does, what species does the natural work of fertilisation.] "Although I have often watched the flowers I have never seen insects directly enter them. It occurred to me, however, that I should be more successful if I were to examine every plant noticed with the lip drawn back against the column. Acting on this idea I soon found three, each enclosing a small dipterous insect. Two of these had no traces of pollen on them, and the flowers were not fertilised. The. third was dead, apparently not having been able to find the passage out of its prison. It had the remains of two pollinia attached to its back. The stigma of the flower was also plentifully covered with pollen, which had evidently been conveyed from another plant, for all four pollen-masses were intact and undisturbed in their cells. "The fact of this insect being unable to effect its escape led me to examine a considerable number of flowers, which had commenced to wither, and in which the sepals and petals had closed together, with the view of ascertaining if this circumstance was of frequent occurrence. The results were important. Out of 119 specimens examined 17 had dead insects, and nine of these insects bore trace* of having had pollen attached to them. Some had followed the passage between the wings of the column until they had reached the anther, and then, becoming glued to the pollen-masses, had not been able to drag them out of their cells, thus perishing on the threshold of their prison. Many of the flowers which did not enclose insects exhibited signs, besides the removal of the pollinia, of having been visited by them, from the nresence of hairs, etc.. adhering to the stigma and rostellum ; and in one instance the antennae of some insect was found glued to the rostellum, proving that its owner had escaped by crawling through the passage in front of that organ. All the insects proved to be Diptera. and are probably referable to one species. What inducement there is to visit the plants I cannot conjecture, for oven with the most careful examination I hare not been able to detect ihe presence of any nectar or nectar-secrctinT organs. "Tire comparatively large number of insects retained in the flowers examined appears, at first sight, to show a serious Imperfection in the contrivances for ensuring fertilisation, as it is evident that it is a loss to the plant when its visitor ennnot escape and carry away the pollinia. On a closer examination, however, it probably only prove- how carefully the passage' for the exit of the insect has

been modified to suit the relative size of the species by which the plant is fertilised, for if the passage had been of a size sufficient to allow the largest individuals to escape with ease the smaller ones would perhaps have been able to pass through without touching the ro'telium, and consequently would not remove the pollinia. "It seldom happens that all the pollinia are removed. (Jut of 110 withered flowers 28 had all the pollinia remaining in their anther cells, 29 had lost one, 34 had lost two, 13 had three withdrawn, while only six had all four removed. Seventy-one of the flowers were fertilised ; but it must not be forgotten that a large number of the unfertilised ones drop off before commencing to wither, so that the proportion fertilised is really much less than this. Probably not one quarter of the flowers ever produce capsules." I think it probable that each species of Pterostylis has a distinct species of fiy to fertilise it, just as is the case with many of the species of Orchis and Ophrys in Britain, where a particular species has its own moth or butterfly. .It is difficult to believe that any flower of Pterostylis is self-fertilised. As all our New Zealand species have solitary fiowors, the cross effected is not only between different flowers but between different plants. All the species which I have hitherto referred to are terrestrial, growing from tuberous roots, and having annual stems. There are also four genera of epiphytic orchids, of which I have examined only two spec;es, Dendrobium cunninghamii and Earina suaveolens. E. mucronata is very common about Ounedin, and should be carefully studied this next season. 1 have looked at one or two flowers of the rather rare and easily-overlooked Sarcochilus adversus, and 'know that it has movable caudicles by which the pollinia are depressed when they are withdrawn from the anther cells, but I have not had sufficient material to work out the mode of fertilisation. Dendrobium cunninghamii has its flowers evidently fitted for cross-fertili-sation. Tho upper sepal is lanceolate in form, and is the smallest in the whorl. The lateral sepals are broad at the base, and adnate to the produced base of the column. The labellum is widely expanded above the middle, with two small lateral lobes, and bears on its face five elevated ridges or plates. It is attached to the base of the column by a short and very elastic claw. The column stands in the flower exactly like the letter J, being produced forward at the base, and terminated by a large green glandular swelling. When the flower is open, a small drop of nectar is always found at the base of this swelling. Tho terminal anther encloses four narrow and flattish pollen masses, attached in pairs to a strap shaped caudiele. The stigmatic surface is placed slightly below it, and is nearly square. When in bud, the lower surface is hollowed into a deep pit, and on its summit, standing directly in front of the bases of the pollinia, is the rostellum, which at this stage is membranous; As the flower opens, the cells of the rostellum become converted into a milky and excessively viscid substance, while the whole surface of the stigma secretes abundantly a clear, viscid matter, and a drop of sweet fluid is secreted at the base of the column.

The action of the parts is exceedingly simple. An insect alighting on the labellum weighs it down very easily, and thus gains access to the nectar at its base. The elasticity of the labellum, however, tends to> keep it pressing against the column, and thus compels the insect to brush against the viscid rostellum. The pollinia are very easily withdrawn by an upward movement, as can be seen by introducing a needle or pencil point, and touching the rostellum in withdrawing it, when one or more of the pollinia will be withdrawn with it. The lateral lobes of the labellum and tho guiding ridges on its surface would prevent an insect reaching the nectar without touching the rostellum when leaving the flower ; and any insect entering another flower with pollinia on its head could not fail to leave these on the stigma. By inserting a fly this action was easily seen, all four pollinia being withdrawn, glued over the insect's right eye. Out of 22 flowers examined., only five had their pollinia removed from the anther cases, but as the plant was growing on a verandah away from its native habitat, this was no criterion. I regret that I did not fertilise any of the flowers on this plant with their own pollen. Those fertilised by pollen from other flowers on the same plant produced fine capsules. Earina suaveolens bears spikes of small very fragrant flowers, which are whitecoloured " with a yellow centre. The plant on which I made my observations flowered in March and April, and had over 1100 blossoms on it. Each flower is only about a-third of an inch in diameter: the' labellum is three-lobed, stands nearly erect in front of the column, and has its lateral lobes produced forward at right angles to it. It bears two strongly marked longitudinal ridges on its surface, which almost touch the sides of the column, and leave a minute passage to its base. There is no nectary, but the tissue at the base of the labellum is easily punctured, and _ exudes beads of moisture. From the position of tho parts it appears to be impossible that self-fertilisation could take place. The four pyriform pollinia are remarkably co he-rent * and lie very closely ensconed in the anther case. For fertilisation by insects, however, the parts are very simply fitted. The rostellum. with the attached caudiele. projects to a small point, and is viscid on the edge and under-surface. An insect visiting the flower would insert its head or proboscis into the small square aperture between the labellum an<T tho column, and in withdrawing would inevitably touch the viscid surface. The slightese touch draws away the pollinia, usually, all four, but sometimes only two. Were they to be withdrawn just as they lie on- the summit of the column thov would hardly be in the position to strike 'the stigmatic surface of another flower ; but in being withdrawn, the cap of the anther pulls them _ slightly downwards and depresses the caudiele con-

siderably. I repeatedly imitated the action with the point, of a pencil or needle, and found in every case that the pollinia came away readily, and were depressed considerably below a right-angle to the surface to which they adhered. In this position they were easily placed on the stigma of a second flower. I carefully examined 91 flowers, and found that the pollinia had been removed from the anthers in 41 cases, and remained intact in 50. Tiie plants were in my garden, and were probably not visited by the insects usual in the bush. 1 have now indicated the modes of fertilisation of most of our native orchids ; but we know very little about the subject yet, and nothing of the insects which are in many cases the active agents. There is therefore plenty of scope for observational work dining the coming season.