In V. odorata (single flower) all five stamens produce pollen. But I daresay all this is known.
LETTER 592. TO J.D. HOOKER. November 3rd {1862}.
Do you remember the scarlet Leschenaultia formosa with the sticky margin outside the indusium? Well, this is the stigma—at least, I find the pollen-tubes here penetrate and nowhere else. What a joke it would be if the stigma is always exterior, and this by far the greatest difficulty in my crossing notions should turn out a case eminently requiring insect aid, and consequently almost inevitably ensuring crossing. By the way, have you any other Goodeniaceae which you could lend me, besides Leschenaultia and Scaevola, of which I have seen enough?
I had a long letter the other day from Crocker of Chichester; he has the real spirit of an experimentalist, but has not done much this summer.
LETTER 593. TO F. MULLER. Down, April 9th and 15th {1866}.
I am very much obliged by your letter of February 13th, abounding with so many highly interesting facts. Your account of the Rubiaceous plant is one of the most extraordinary that I have ever read, and I am glad you are going to publish it. I have long wished some one to observe the fertilisation of Scaevola, and you must permit me to tell you what I have observed. First, for the allied genus of Leschenaultia: utterly disbelieving that it fertilises itself, I introduced a camel-hair brush into the flower in the same way as a bee would enter, and I found that the flowers were thus fertilised, which never otherwise happens; I then searched for the stigma, and found it outside the indusium with the pollen-tubes penetrating it; and I convinced Dr. Hooker that botanists were quite wrong in supposing that the stigma lay inside the indusium. In Scaevola microcarpa the structure is very different, for the immature stigma lies at the base within the indusium, and as the stigma grows it pushes the pollen out of the indusium, and it then clings to the hairs which fringe the tips of the indusium; and when an insect enters the flower, the pollen (as I have seen) is swept from these long hairs on to the insect's back. The stigma continues to grow, but is not apparently ready for impregnation until it is developed into two long protruding horns, at which period all the pollen has been pushed out of the indusium. But my observations are here at fault, for I did not observe the penetration of the pollen-tubes. The case is almost parallel with that of Lobelia. Now, I hope you will get two plants of Scaevola, and protect one from insects, leaving the other uncovered, and observe the results, both in the number of capsules produced, and in the average number of seeds in each. It would be well to fertilise half a dozen flowers under the net, to prove that the cover is not injurious to fertility.
With respect to your case of Aristolochia, I think further observation would convince you that it is not fertilised only by larvae, for in a nearly parallel case of an Arum and a Aristolochia, I found that insects flew from flower to flower. I would suggest to you to observe any cases of flowers which catch insects by their probosces, as occurs with some of the Apocyneae (593/1. Probably Asclepiadeae. See H. Muller, "Fertilisation of Flowers," page 396.); I have never been able to conceive for what purpose (if any) this is effected; at the same time, if I tempt you to neglect your zoological work for these miscellaneous observations I shall be guilty of a great crime.
To return for a moment to the indusium: how curious it is that the pollen should be thus collected in a special receptacle, afterwards to be swept out by insects' agency!
I am surprised at what you tell me about the fewness of the flowers of your native orchids which produce seed-capsules. What a contrast with our temperate European species, with the exception of some species of Ophrys!—I now know of three or four cases of self-fertilising orchids, but all these are provided with means for an occasional cross.
I am sorry to say Dr. Cruger is dead from a fever.