This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Read before the American Association for the Advancement of Science, at the meeting at Saratoga, by Thomas Meehan, Fellow of the Association.
In the transactions of the Academy of Science of St. Louis of April 15, 1873, our distinguished associate Dr. George Englemann has some "Notes on the genus Yucca," in which occurs the following passage: " The conspicuously papillose termination of the pistil had always been considered the stigma, but closer examination showed its papillae to be epidermal appendages, corresponding to similar ones on the filaments, and entirely destitute of stigmatic function; never did they contribute to the development of a pollen grain occasionally adhering to them. Dr. Mellichamp's notice of a drop of glutinous liquid in the tube formed by the coalescence of the so-called stigmas led me to further experiments. That tube proved to be the real stigma, exuding stigmatic liquor, and insects must be the agents which introduced the pollen into the tube " Subsequent investigations by our esteemed associate Professor Riley led to the discovery of a new genus of Lepidoptera - Pronuba yuccasella - and which has proved to be the insect agent which fertilizes the flower.
In the same number of the Proceedings, Professor Riley describes this insect and says, " with her maxilary tentacle, so wonderfully modified for the purpose, she collects the pollen in large pellets, and holds it under the neck and against the front trochanters. In this manner she sometimes carries a mass twice the size of her head. Thus laden she clings to the top of the pistil, bends her head, thrusts her tongue into the stigmatic nectary and brings the pollen masses right over its mouth. In this position she works with a vigor that would indicate combined pleasure and purpose - moving her head and body from side to side, and apparently making every effort to force the pollen into the tube. Such is the method by which our yuccas are fertilized".
It may be remembered that at our meeting at Buffalo I produced three capsules that had not been produced by this elaborate process, but simply by mere touching of the papillose apex with one of the flower's own polleniferous anthers. Professor Riley was so sure that the seed-vessels could not have been produced in that way; that there must have been some insect agency unknown to me in addition to my work, that at the conclusion of my paper he asked permission to cut open the capsules, sure of being able to show the larvae in the fruit; but he found them not. I recall these matters to show that I have not misapprehended the position our friends take on this question.
I now again exhibit numerous seed vessels from this plant of Yucca angustifolia in which no trace of larva? can be found; and seed vessels of Yucca filamentosa growing but a few yards from the other, which are infested by the Pronuba yuccasella, as this species always is when it seeds at all.
The history of the fruiting of the Yucca angustifolia is as follows: It flowered in 1875, but produced no fruit.* In 1876 the early flowers proving infertile, I applied the flower's own pollen to the apex of the pistil of the four last flowers that opened; these produced the four capsules examined by Professor Riley as already noticed. In 1877, noticing that the Pronuba abounded in the flowers, no hand application was made, and there was no fruit. In 1878 the flowers were again left to the insects with no fruitful results. The past season pollenization by hand was resorted to, and the numerous seed vessels I exhibit followed. As the pollen was merely applied to the papillose apex it shows that in this species the elaborate and wonderful ingenuity of the insect in applying pollen as described by our friend is wholly unnecessary.
We now come to some extremely interesting considerations, growing out of these facts.
Pronuba yuccasella, the yucca moth, has for years abounded on my flowers of the Yucca filamentosa. It has not been known to visit any other plant than yucca. Yucca angustifolia begins to flower from two to three weeks, and its blossoming is all over before Yucca filamentosa begins to open. The facts now adduced show that the moths exist weeks before the flowers bloom with which they have been so intimately connected, feeding of course on other flowers, and would perhaps make use of other fruits as depositaries for their eggs if yucca should not exist. At any rate, the facts weaken any belief we may have that the yucca and all yucca moth through the long ages have become mutually adapted to each other through a fancied mutual benefit.
* At the. conclusion of this address, delivered at the Saratoga meeting of the American Association, Prof. C. V. Riley made some remarks which unfortunately I did not hear. The newspaper reports make him say that I was mistaken in the insect I found in Yucca angustifolia, that it was not Pronuba yuccasella. 1 have called Prof. Riley's attention to this, and have asked for a correct note of what he did say, but have only the reply that he is " not answerable to a newspaper report." It remains then only for me to say in reply to the " newspaper report" that at the outset of my observations on Yucca angustifolia, I sent one of the insects caught to Professor Riley asking: "Is this certainly Pronuba uccacella?" and he replied that it was.
But the fact remains that the yucca is so arranged that it must have external aid before it can use pollen; and it is believed that this arrangement is for the express purpose of facilitating the introduction of strange pollen; and further that this arrangement must be useful, or it would not exist. And then it is assumed that this useful purpose can only be understood by believing that cross-fertilized seed is of the most benefit to the race. Let us examine this reasoning in the light of facts: -
In 1871, I found Yucca angusti/olia seeding abundantly in Colorado; but when the interesting matters in its history were brought out by Professor Riley I could not remember whether the seed vessels were infested by the larvae of the yucca moth, and was glad to revisit Colorado in 73 to examine the plants, but I did not find one seed vessel in several weeks' search for them. I have since engaged friends to get me some, but none have found them. In order to test the matter thoroughly I engaged with a professional seed collector in Southern Utah to buy of him a pound of seed of each of the several species, and recently I have heard from him - the third successive year - that no plant within his observation has produced a single seed. How can we believe that this elaborate arrangement for producing seeds by cross-fertilization through insect agency is for the purpose of producing a better class of seeds, when we see in many cases plants utterly fail, even for successive years, to seed at all?
We know that it is not to the interest of the individual to produce any seeds. Seed is a pro- rision of nature looking to the good of the future, and to which the present good of the individual is often sacrificed. The mignonette, the petunia, the gaillardia, and many other plants under garden culture live for years when prevented from perfecting seeds. We may fairly believe that a plant which acquires the power of easy individual increase and persistence would show less disposition to sacrifice itself on seed; and thus we do find in nature that it is among that class which has the most of this individual persistence that the indifference to self-fertilized seeds, popularly known as "arrangements for cross-fertilization," is found, and moreover that the most difficulty in germinating is met with even when the seed is freely formed.
The yucca, by its large fleshy root stocks and ability to withstand extremes of drought and heat and cold, is able to maintain an existence indefinitely without producing any seed. For the sake of inducing variation, which is best accomplished through seeds, it may be compelled by inexorable law once in a while to produce them; but a law which is to result in the evolution of new forms will hardly be adduced in favor of any theory which has for its foundation the idea of benefit to an existing race.
[It was our intention to give this to our readers earlier, so as to lead them to observe yuccas this season; but have given place to the favors of correspondents. - Ed. G. M.|