From George Henslow 6 November 1865
10 South Crescent | Bedford Sq: W.C.
My dear Sir,
I beg to thank you very much for yr kind & instructive letter:1 there are one or two passages in it which,—with your permission I should like to read in connection with a few notes I purposing giving at the next Linnean Meeting.2
I should much like to read Sprengels account of Salvia, & am glad to find my surmise to have been corroborrated by yourself. (Would you kindly give me the reference to Sprengls paper?)3 With regard to Medicago, perhaps the word Irritability is not exactly the true one. Elasticity also does not express the facts which are these. The stamens are at first straight & horizontal included within the alæ & Carina; the former of which have little processes overlying the stamens & pressing down upon them: this, they are able to do by being firmly locked with the carina.4
The instant any object, e.g. a pin, is directed down the channel of the Vexillum, it cannot help pressing upon these “processes” laterally. The stamens then spring up & the carina spreads out &, with the alæ, drop. But the stamens not only rise up but immeadiately become curved towards the Vexillum in the form of a circle; so that there seems to me to be some other force required, besides mere elasticity, especially as this would imply the possibility of making them return to their original horizontal position: which they will not do, but crack transversely if it be attempted. Having however once assumed their ‘curved’ ‘rigid’ & ‘vertical’ position it cannot be altered.5
I have read Cohn’s paper & have been much interested in it, especially as I had been, also, examining some Centaureas,6 & without knowing anything had been previously observed, thought I saw some peculiarity, but I found it was simply elasticity. I did not discover the independent irritability.7 My own conclusion was this;—that as the style rises & stretches the anthers to their full extent by pushing against their closed valvate extremities, until it succeeds in forcing its way through,—so on the other hand, the instant the style is through, the tension of the filaments is relaxed & the anthers are drawn down again. So that two opposing forces are brought into play, the object being to enable the ring of hairs (if present) more effectually to sweep out the pollen, & so elevate on the outside of the two stigmatic ends which now & not till now, separate.
I presumed all this was intimately connected with intercrossing.8
Many thanks for your kind offer of giving me further information; which, I should like to avail myself off at another time.
Yours very truly | Geo. Henslow
P.S. I beg to thank you for the printed matter which I will return9
Pleased CD confirms his observations on Salvia.
Spring action of Medicago stamens described.
Please cite as
Darwin Correspondence Project, “Letter no. 4931,” accessed on 20 January 2017, http://www.darwinproject.ac.uk/DCP-LETT-4931