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Letter 10566

Breitenbach, Wilhelm to Darwin, C. R.

26 July 1876

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    Observations on pollinia of Orchis maculata

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    and on Primula elatior. [On latter, see Forms of flowers, p. 34.]

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[Enclosure: 1]

Concerning the movement of the Pollinia in Orchis Maculata

If one puts a pointed pencil or a needle into the flower of our meadow orchis, in the same way as a humble-bee inserts its trunk, the pencil touches the Rostellum. The skin of the Rostellum breaks and the pencil comes in contact with the Klebscheibchen of the Pollinia which on account of their sticky condition become attached to the pencil, so that if one withdraws the pencil from the flower the Pollinia stick to it. The Pollinia then generally form something like a right angle with the pencil. When the Klebscheibchen of the Pollinia have been for a short time exposed to the air the Pollinium sinks towards the point of the pencil till it lies close to it. If one with a pencil prepared in this way enters a second flower the point of the Pollinium meets the stigma. It is clear therefore that this downward movement of the Pollinia is absolutely necessary for the fertilization of the flower. Mr. Darwin thinks to have explained this downward movement of the Pollinia through a contraction of the Klebscheibchen caused by the drying up of its cells.

If the Pollinium really forms a right angle with the pencil the contemporary contraction of the whole Klebscheibchen by no means explains the operation. For why should the Pollinium always fall in one and the same direction, why should it not sometimes fall sideways, or backwards? or why does it not remain upright? But if the Pollinium does not form a right angle with the pencil, if on the contrary the angle between it and the point of the pencil is an acute one, the corresponding angle being obtuse, then the process is satisfactorily explained by an equal contraction of the whole Pollinium. If, this being the case, the cells are contracted, the Pollinium must necessarily, in consequence of the weight of its head, sink towards the point of the pencil. If the pencil is inserted into the flower according to the rule laid down, the pencil being held perpendicular to the observer, it is evident from the structure of the flower, that the Pollinia can only form right angles with the pencil. The Pollinia must therefore sink down in a very decided direction. It is true that according to my observations, the Pollinium does not seem to form a right angle with the pencil (See Fig 1) With this modification Mr. Darwin's explanation seems to be correct. I maintain however that the movement of the Pollinia is not explained by the equal contraction of the ``Kle<b>scheibchen'', but on the contrary by an unequal contraction, which beginning at that part of the ``Scheibchen'' which lies towards the point of the pencil, extends gradually towards the opposite side, but so that when the contraction has already progressed beyond the middle of the ``Klebscheibchen'', the first half is not yet completely shrivelled up. It is in consequence of a number of observations which I made on Orchis Maculata this summer that Mr Darwin's explanation seems to me inefficient.—

On drawing the Pollinia out of the flower, I found in very many cases, that they had not the usual position, (1) but that they were bent backwards and were lying close to the pencil. (3) But they by no means remained in this position, but moved with tolerable quickness, (namely in 10 seconds,) upwards, until they had attained the position they usually occupy (4). I further observed, to my great astonishment, that in by far the most numerous cases, in which the Pollinia behaved in this unusual way (3), they remained in that upright position, and did not sink down, so as to take the position suitable for fertilization. It is clear that my observations do not agree with Mr. Darwin's explanation. Even if we accept the modification of Mr. Darwin's explanation I have pointed out (The angle made by the inclination of the Pollinium towards the point of the pencil being an acute one), my observations are not thus explained. On the other hand, as we shall see directly, my observations are explained easily by my supposition of a gradually progressing contraction of the K. from the front backwards. And as my hypothesis of contraction likewise explains the normal movements of the Pollinia in a perfectly satisfactory manner, my explanation (supposing that I am right in my assumption deserves to be preferred to Mr. Darwin's, if only because it is more comprehensive than Mr. Darwin's; for a hypothesis becomes the more probable, the greater the number of facts which can be mechanically explained thereby. I have therefore now to explain 1 The upward movement of the Pollinia; 2 The standing still of the Pollinia in that (upward) position. 3. The normal downward movement of the Pollinia.

On being withdrawn from the flower the Pollinia lie abnormally turned backwards on the pencil. When the Klebscheibchen is exposed to the air its cells contract, beginning in front. In consequence the neighbouring cells are little by little drawn towards the front and therewith also the cells of the Caudiculum of the Pollinium, so that the latter must be raised. As the contraction of the cells of the ``Klebscheibchen'' increases towards the middle, the Pollinium also will be raised increasingly.

When the contraction has reached the middle of the ``Klebscheibchen'', the Pollinium stands perpendicularly on the ``Scheibchen'', and therefore also perpendicularly on the pencil. If the contraction did not proceed beyond the middle of the Klebscheibchen ? or instead of progressing gradually, did so by jerks, so that at about the middle of the Scheibchen one of the periods of contraction were to come to an end, then the Pollinium might next sink down towards the point of the pencil. But if the contraction progresses gradually, this cannot happen. The very moment, when the Pollinium reaches the position which would enable it to fall towards the point of the pencil, an opposing force is added which draws it to the opposite side. The Pollinium has been raised through the contraction of that part of the ``Klebscheibchen'' which lies in front of the Caudiculum, it would now sink towards the point of the pencil, if the contraction of that part of the ``Klebscheibchen'' which lies behind the Caudiculum, did not draw it back towards the very side whence it was first raised. If the front part of the ``Klebscheibchen'' were to be thoroughly shrivelled up, at the time when the opposite half was only beginning to shrink, the Pollinium would necessarily be drawn back into its first position. But as the contraction of the further side of the ``Klebscheibchen'' takes place while in the front part contraction is still going on, both contractions as it were balance each other. It would appear indeed that nevertheless the contraction of the farther half must outweigh that of the front part, so that the Pollinium would be somewhat drawn backwards. But we must not forget that the cells of the Caudiculum which lie close to the ``Klebscheibchen'' also shrivel up, and that this must interfere considerably with the moveability of the Caudiculum.

I have to add a few words in explanation of the normal movement of the Pollinium. If the Pollinium on being withdrawn from the flower stands in the usual position (1.) it will as the contraction proceeds sink down, until on the contraction having reached the middle of the ``Scheibchen'', it lies against the point of the pencil (2) Thus both in the normal and in the abnormal cases, the Pollinia are drawn 90 degrees out of their position. In the normal cases the like process takes place as in the abnormal. If when the contraction begins in that half of the ``Klebscheibchen'' which is turned away from the point of the pencil, the contraction were to cease entirely in the opposite half, the Pollinium would be drawn back again through 90 degr: But as, while the farther side of the Klebscheibchen is being gradually contracted, the opposite side is not yet thoroughly shrunk, but on the contrary still possesses some strength to pull, which is exerted to move the Pollinium forward; and as the Caudiculum has lost much of its moveability in consequence of the shrinking up of its lower cells, the Pollinium will hardly be perceptibly moved from its position after it has travelled through 90 degrees. As to those cases in which the Pollinium does not move through 90. degrees, but through a much smaller distance (such cases are not uncommon) the explanation is simple enough. In those cases the contraction of the ``Klebscheibchen'' and the contraction of the lower cells of the Caudiculum perhaps likewise, has taken place so quickly, that the Pollinium has not had time to move through 90 degrees. I have been convinced by my observations on Orchis Maculata that such cases happen both with the normal and abnormal movements of the Pollinia | |Wilhelm Breitenbach | Lippstadt July 11th. | 1876.

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