Roy. Hort. Soc. 27: 852-856 (1903)
GERMINATION IN AMARYLLIDS
By A. WORSLEY, F.R.H.S.
THE fig. 191, B, I have inserted to show that the apex of the original process issuing from the seed of Hymenocallids is not always simple. All drawings I have seen depict a single root in prolongation of the alleged cotyledon. Such is by no means always the case. Often Crinum Moorei seeds will show no signs of any root whatever until the young bulb is formed and the tissue of the seed quite dead. Shortly after this time (perhaps three months after germination) a number of roots will issue simultaneously. Even if it is true that at the moment of emission from the seed the original process is always simple, yet it may evidently become branched at a very early stage in its career, or become abortive. Yet in many Liliaceae, many annuals, Palms, &c., this first root becomes one of the vital organs of the mature plant.
Original position of bulb (now dead).
Level of new growing point, or top of new bulb.
|FIG. 190.—ISMENE HYBRIDA "SULPHUREA," SHOWING BULB-EXTENSION.|
In fig. 191, C (Ismene calathina), the termination of this process has, five months after germination, become a true root, and shows no sign of decay. No leaves whatever have yet been emitted, but the seed is dead, and a true bulb left alive. Considerable loss of weight has resulted. Ten seeds weigh one ounce, whereas it would take eighteen bulbs of this size to do so.
A.—Hymenocallis concinna [Baker, sp. nov.], showing seed a month after sowing. Two processes have issued from one seed, and are both forming bulbs, showing that two embryos may exist in one seed of this species.
B.—Another seed of same fruit showing two roots issuing during weaning Period. The tissue of seed was not visibly atrophied at this period.
C.—Young bulb of Ismene calathina five months after germination. The seed is dead. No leaf-growth has taken place yet.
This loss of weight will continue until foliation begins, by which time the bulb will only weigh a quarter of the seed from which it sprang.
The drawing, fig. 191, A, shows that it is possible for two embryos to exist in one seed of Hymenocallis concinna [Baker, sp. nov.] Among the Amaryllids I have never previously noticed such an occurrence, and I believe it to be unique in the literature of these plants.
The arrangement of the ovules in Hymenocallis concinna is quite typical of the genus as described by Bentham and Hooker—a single pair of ovules lying at the base of each cell, or six ovules in all to each ovary.
The minute threads by means of which the ovules adhere to the placenta, and which constitute the only direct communication with the stigma, would seem to be so arranged that the whole of the ovules would become impregnated contemporaneously, on the adhesion of a sufficient number of virile pollen grains to the stigma.
Hence there does not appear at first sight to be any reason why each ovule should not produce a seed. Yet it is a fact that such sequence does not occur in the Amaryllids with bulbiform seeds. Such plants have a fixed number of ovules and a fixed number of seeds (subject to very small fluctuation); yet there is often a great dissimilarity between the number of seeds and of ovules.
In this Hymenocallis the difference is not great, four seeds being produced by the six ovules in the few fruits I have raised.
Although I believe that every ovule is impregnated simultaneously, yet the possibility of seed production must be held to be limited, and I believe that we must seek in the structure of the tissues below the ovary the cause of such limitations.
It would seem as though the carriage of nutriment to the embryo from the bulb of the parent was either limited by the structure of such tissues to a certain number of channels, or that all the nutriment became, after impregnation, quickly diverted to the strongest embryos, and that the rest suffered from strangulation or starvation. The appearance of the seeds in certain Crinums, Hippeastrums, &c. lends weight to this supposition, for we find in the same fruit seeds of varying sizes; some of great vitality, others of less; some in which the embryo is just alive, strong enough to germinate, but not to carry it through the weaning period; others in which the germ is already dead, but yet has evidently grown at one time out of the ovule stage.
If anyone feels inclined to deny the contemporaneous impregnation of all the ovules in one ovary, he is met by this difficulty, that if it were possible to effect partial impregnation it would also appear possible to effect diverse impregnation in the same flower; that is to say, that species "A," known to be virile under the pollen of species "B" and of species "C," could have one lobe of a stigma impregnated with "B" pollen, and another lobe with "C " pollen, and produce, in the same fruit, seeds of diverse hybrids.
My own experiments have satisfied me that in every case impregnation of all the ovules is contemporaneous. I have very often mixed the pollen from many Amaryllids and applied it to a stigma, but never with the result of effecting diverse impregnation.
From this it would appear as though a single pollen grain were capable of impregnating over one hundred ovules, such as exist in the ovary of Hippeastrum. Yet I do not think we are justified in asserting this to he so ascertained fact.
Certainly I have noticed that in making inter-generic and inter-specific pollenisations the number of seeds produced has always been much below the normal number; that in the vast majority of cases no seeds were produced, the embryos having died, although evidence of their having lived and having passed beyond the ovule stage was in many cases incontestable; in other cases there were no evidences of impregnation.
These experiments tend to suggest that, beyond the one act of excitation, or impregnation, of the female germs, there remains some further function for the male germs to perform, which is improperly, or only partially, done by foreign pollen grains.
To revert again to the seed of Hymenocallis concinna (fig. 191, A). In this instance the original process issuing from the seed is duplicated, and each process is terminated by a bulb in process of formation. I can only account for this by admitting the existence of two embryos within the one seed. These again can only trace their existence to the incidence of two germs within the one ovule from whence the seed sprang. Yet if two germs can exist within the one ovule, why not more than two ? Is Nature bound never to exceed the assigned number, or may we contemplate the possibility of erraticism? And if we have this direct proof that in these Amaryllids more than one female germ may exist within one ovule, we must not exclude the possibility of the multiplication of male germs within the pollen grains. Nor can we altogether exclude the possibility that the male germ, in the process of absorptive growth which ensues when it is brought into contact with the saccharine matter of the stigma, and in the course of the somewhat obscure chemical changes which result, may not become increased by subdivision, or by gemmation, into numerous active units. Perhaps this may give an explanation of hew one pollen grain may be able to impregnate many ovules.
To take a simile from chemistry, may we seek in the molecule and the atom, or in their organic counterparts, the wherewithal to construct the most probable, or the least improbable, theory reconcilable with facts no longer subject to direct and tangible proofs?
I have noticed in the direction taken by the original process issuing from the bulbiform seeds of Crinum &c. a very adaptive sequence of events to ensure this process reaching the ground.
The shape of the seeds is such that it is impossible to foresee what side will, upon dehiscence from the fruit, ultimately rest upon the ground, and, as was pointed out in the first instance, I believe, by Salisbury, there appears to be no law governing the point of emergence of the original process from the bulbiform seed. Hence the same might be upwards or sideways, and the process might end, and the bulb be formed, in some position whence the roots (when emitted) might be unable to reach the mother earth.
Those persons who recline upon the theory of gravitation to get them over every difficulty connected with the direction of root growth account for the curvature of this original process in their own way, oblivious of the fact that, whatever this original process may be, its principal function is not that of a root.
Yet we need not go so far for an explanation when one is ready to hand in progressive adaptation.
It must be evident that plants having such seeds and emitting a straight process would tend to become extinct, whereas the first one which emitted a curved process would, in whatever position the seed fell, shortly bury itself in the ground.
Hence the doctrine of "the survival of the fittest" is alone sufficient to account for the more perfectly adapted type of seed-producing plant outliving the imperfectly adapted type.
And by the slow steps of progressive adaptation we may trace the path leading up to this perfect and beautiful sequence of growth to which I have on other occasions drawn your attention.
J. Roy. Hort. Soc. 27: 956-957 (1903)
BULB-EXTENSION IN DECIDUOUS ANDINE AMARYLLIDS
By A. WORSLEY, F.R.H.S.
AMONG the following genera and sub-genera indigenous to the Andes, viz. Ismene, Elisena, Chlidanthus, and Habranthus [and probably also in Placea, Phycella, and Eucrosia], certain characteristic bulb-growth has been noticed by various writers. None of these writers have, as far as I am aware, given any comprehensive explanation of the facts which they had noted. In place of this we find rather disjointed notes scattered over a vast literary field, and in fact many mutually destructive theories have been advanced to explain what some considered abnormalities.
Other authors—and these are the majority—take the very superficial view that such bulbs are drawn underground by the action of their "contractile" roots. Without denying that the roots of these bulbs may exercise a special function of this kind, yet I am convinced that we should not seek here for the sole, or even the main, cause of subsidence. Such bulbs begin to bury themselves before they have any true roots, and continue to do so after, but not solely because, root action has begun.
The life-history and race-history of these plants show that they sustain a constant struggle to bury themselves in the soil.
On germination of the seed the original process (the cotyledon of some writers) transposes the embryo from the seed and causes it to undergo the process of weaning often half a foot deep in the ground. (See p. 853, fig. 191.)
The young bulb, perhaps during its first deciduous period, certainly every year after attaining maturity, loses by decay the whole upper portion of the bulb. Simultaneously a still greater growth and swelling occurs in the lower part of the bulb.
In the full-size drawing here given the upper half of the bulb is already dead, and the point of issue of the new leaf-growth will be from the constricted middle (or waist) of the bulb upward through a crater-shaped hole in the dead upper part of the bulb (which was filled in the year previous by the bases of the leaves).
The top of the bulb maintains in the pot its original level, thus showing that, in this case at least, the bulb has not been drawn downwards by "contractile" roots, but the great growth in the lower part of the bulb has in reality formed a new bulb below the original one, and it would appear as though the only part of the plant which persisted from year to year was the root-stock (disc). When leaf-growth recommences and the soil becomes moist the dead upper portion of the bulb will speedily disappear and perish.
It is very easy to trace this regular process of events in gardens where Ismenes or Elisenas are grown in pots. For to keep them in good health it is almost necessary to repot them all in autumn, at which time every bulb will be found near the bottom of the pot, with the dying old bulb-tunics in various stages of decay above it.
With Habranthus and Chlidanthus much variation will be found in the shapes of the bulbs: some will have buried themselves more than others, some not appreciably. Seasons also appear to influence the burying propensities of these latter plants. There are writers who place much importance upon the power of the roots to clasp large stones &c., and by their growth draw down the bulb through the soil.
It is evident to me, however, that it is not necessary, in the cases of the plants with which I am dealing, to have recourse to this theory. The yearly growth downwards and laterally of the lower part of the bulb and the yearly death of the upper pert account quite satisfactorily for all the facts that have been noted concerning them.
The fact that I selected a hybrid Ismene to draw from was because it happened to present at the time a typical instance of growth; not more so, however, than either parent exhibits at the same season.
A great part of the Andine regions are situated in zones of "dry and wet seasons." In many cases the soil is easily disintegrated and the gradients very steep.
On such steep hillsides the heavy and sudden rains falling on desiccated soil cause great detrition, and any bulb lying on the surface is liable to be washed away. By establishing themselves at a deeper level some immunity from such accidents is attained. The bulbs also avoid the extremes of surface temperature both in summer and winter, the attacks of hungry animals, and countless other chances to which surface bulbs must be exposed, not to mention the possibility of being started into growth by any slight showers of rain during the dry season.
Possibly there may be other advantages gained by these self-burying plants.