TABLE 4.26.
Column 1: Nature of union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Average Number of Seeds per Capsule.
Column 5: Average Number of Seeds per Flower fertilised.
The six legitimate unions : 75 : 56 : 96.29 : 71.89.
The twelve illegitimate unions : 146 : 36 : 44.72 : 11.03.
Therefore the fertility of the legitimate unions to that of the illegitimate, as judged by the proportion of the fertilised flowers which yielded capsules, is as 100 to 33; and judged by the average number of seeds per capsule, as 100 to 46.
From this summary and the several foregoing tables we see that it is only pollen from the longest stamens which can fully fertilise the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil. And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect"s body which becomes charged with the proper pollen. It is also evident that the stigma of each form, fertilised in three different ways with pollen from the longest, mid-length, and shortest stamens, is acted on very differently, and conversely that the pollen from the twelve longest, twelve mid-length, and twelve shortest stamens acts very differently on each of the three stigmas; so that there are three sets of female and of male organs. Moreover, in most cases the six stamens of each set differ somewhat in their fertilising power from the six corresponding ones in one of the other forms. We may further draw the remarkable conclusion that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilisation, by so much is the sterility of the union increased. There are no exceptions to this rule. To understand what follows the reader should look to Tables 4.23, 4.24 and 4.25, and to the diagram Figure 4.10. In the long-styled form the short stamens obviously differ in length from the pistil to a greater degree than do the mid-length stamens; and the capsules produced by the use of pollen from the shortest stamens contain fewer seeds than those produced by the pollen from the mid-length stamens. The same result follows with the long-styled form, from the use of the pollen of shortest stamens of the mid-styled form and of the mid-length stamens of the short-styled form. The same rule also holds good with the mid-styled and short- styled forms, when illegitimately fertilised with pollen from the stamens more or less unequal in length to their pistils. Certainly the difference in sterility in these several cases is slight; but, as far as we are enabled to judge, it always increases with the increasing inequality of length between the pistil and the stamens which are used in each case.
The correspondence in length between the pistil in each form and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of high service to the species by leading to full and legitimate fertilisation. But the rule of the increased sterility of the illegitimate unions according to the greater inequality in length between the pistils and stamens employed for the union can be of no service. With some heterostyled dimorphic plants the difference of fertility between the two illegitimate unions appears at first sight to be related to the facility of self-fertilisation; so that when from the position of the parts the liability in one form to self- fertilisation is greater than in the other, a union of this kind has been checked by having been rendered the more sterile of the two. But this explanation does not apply to Lythrum; thus the stigma of the long-styled form is more liable to be illegitimately fertilised with pollen from its own mid- length stamens, or with pollen from the mid-length stamens of the short-styled form, than by its own shortest stamens or those of the mid-styled form; yet the two former unions, which it might have been expected would have been guarded against by increased sterility, are much less likely to be effected. The same relation holds good even in a more striking manner with the mid-styled form, and with the short-styled form as far as the extreme sterility of all its illegitimate unions allows of any comparison. We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens, is a purposeless result, incidental on those changes through which the species has pa.s.sed in acquiring certain characters fitted to ensure the legitimate fertilisation of the three forms.
Another conclusion which may be drawn from Tables 4.23, 4.24, and 4.25, even from a glance at them, is that the mid-styled form differs from both the others in its much higher capacity for fertilisation in various ways. Not only did the twenty-four flowers legitimately fertilised by the stamens of corresponding lengths, all, or all but one, yield capsules rich in seed; but of the other four illegitimate unions, that by the longest stamens of the short-styled form was highly fertile, though less so than the two legitimate unions, and that by the mid-length stamens of the long-styled form was fertile to a considerable degree; the remaining two illegitimate unions, namely, with this form"s own pollen, were sterile, but in different degrees. So that the mid-styled form, when fertilised in the six different possible methods, evinces five grades of fertility. By comparing Tables 4.24.3 and 4.24.6 we may see that the action of the pollen from the shortest stamens of the long-styled and mid-styled forms is widely different; in the one case above half the fertilised flowers yielded capsules containing a fair number of seeds; in the other case not one capsule was produced. So, again, the green, large-grained pollen from the longest stamens of the short-styled and mid-styled forms (in Tables 4.24.4 and 4.24.5) is widely different. In both these cases the difference in action is so plain that it cannot be mistaken, but it can be corroborated. If we look to Table 4.25 to the legitimate action of the shortest stamens of the long- and mid-styled forms on the short-styled form, we again see a similar but slighter difference, the pollen of the shortest stamens of the mid-styled form yielding a smaller average of seed during the two years of 1862 and 1863 than that from the shortest stamens of the long-styled form. Again, if we look to Table 4.23, to the legitimate action on the long-styled form of the green pollen of the two sets of longest stamens, we shall find exactly the same result, namely, that the pollen from the longest stamens of the mid-styled form yielded during both years fewer seeds than that from the longest stamens of the short-styled form. Hence it is certain that the two kinds of pollen produced by the mid-styled form are less potent than the two similar kinds of pollen produced by the corresponding stamens of the other two forms.
In close connection with the lesser potency of the two kinds of pollen of the mid-styled form is the fact that, according to H. Muller, the grains of both are a little less in diameter than the corresponding grains produced by the other two forms. Thus the grains from the longest stamens of the mid-styled form are 9 to 10, whilst those from the corresponding stamens of the short-styled form are 9 1/2 to 10 1/2 in diameter. So, again, the grains from the shortest stamens of the mid-styled are 6, whilst those from the corresponding stamens of the long- styled are 6 to 6 1/2 in diameter. It would thus appear as if the male organs of the mid-styled form, though not as yet rudimentary, were tending in this direction. On the other hand, the female organs of this form are in an eminently efficient state, for the naturally fertilised capsules yielded a considerably larger average number of seeds than those of the other two forms--almost every flower which was artificially fertilised in a legitimate manner produced a capsule--and most of the illegitimate unions were highly productive. The mid- styled form thus appears to be highly feminine in nature; and although, as just remarked, it is impossible to consider its two well-developed sets of stamens which produce an abundance of pollen as being in a rudimentary condition, yet we can hardly avoid connecting as balanced the higher efficiency of the female organs in this form with the lesser efficiency and lesser size of its two kinds of pollen-grains. The whole case appears to me a very curious one.
It may be observed in Tables 4.23 to 4.25 that some of the illegitimate unions yielded during neither year a single seed; but, judging from the long-styled plants, it is probable, if such unions were to be effected repeatedly by the aid of insects under the most favourable conditions, some few seeds would be produced in every case. Anyhow, it is certain that in all twelve illegitimate unions the pollen-tubes penetrated the stigma in the course of eighteen hours.
At first I thought that two kinds of pollen placed together on the same stigma would perhaps yield more seed than one kind by itself; but we have seen that this is not so with each form"s own two kinds of pollen; nor is it probable in any case, as I occasionally got, by the use of a single kind of pollen, fully as many seeds as a capsule naturally fertilised ever produces. Moreover the pollen from a single anther is far more than sufficient to fertilise fully a stigma; hence, in this as with so many other plants, more than twelve times as much of each kind of pollen is produced as is necessary to ensure the full fertilisation of each form. From the dusted condition of the bodies of the bees which I caught on the flowers, it is probable that pollen of various kinds is often deposited on all three stigmas; but from the facts already given with respect to the two forms of Primula, there can hardly be a doubt that pollen from the stamens of corresponding length placed on a stigma would be prepotent over any other kind of pollen and obliterate its effects,--even if the latter had been placed on the stigma some hours previously.
Finally, it has now been shown that Lythrum salicaria presents the extraordinary case of the same species bearing three females, different in structure and function, and three or even five sets (if minor differences are considered) of males; each set consisting of half-a-dozen, which likewise differ from one another in structure and function.
[Lythrum Graefferi.
I have examined numerous dried flowers of this species, each from a separate plant, sent me from Kew. Like L. salicaria, it is trimorphic, and the three forms apparently occur in about equal numbers. In the long-styled form the pistil projects about one-third of the length of the calyx beyond its mouth, and is therefore relatively much shorter than in L. salicaria; the globose and hirsute stigma is larger than that of the other two forms; the six mid-length stamens, which are graduated in length, have their anthers standing close above and close beneath the mouth of the calyx; the six shortest stamens rise rather above the middle of the calyx. In the mid-styled form the stigma projects just above the mouth of the calyx, and stands almost on a level with the mid-length stamens of the long and short-styled forms; its own longest stamens project well above the mouth of the calyx, and stand a little above the level of the stigma of the long-styled form. In short, without entering on further details, there is a close general correspondence in structure between this species and L.
salicaria, but with some differences in the proportional lengths of the parts.
The fact of each of the three pistils having two sets of stamens of corresponding lengths, borne by the two other forms, comes out conspicuously. In the mid-styled form the pollen-grains from the longest stamens are nearly double the diameter of those from the shortest stamens; so that there is a greater difference in this respect than in L. salicaria. In the long-styled form, also, the difference in diameter between the pollen-grains of the mid-length and shortest stamens is greater than in L. salicaria. These comparisons, however, must be received with caution, as they were made on specimens soaked in water after having been long kept dry.
Lythrum thymifolia.
This form, according to Vaucher, is dimorphic, like Primula, and therefore presents only two forms. (4/8. "Hist. Phys. des Plantes d"Europe" tome 2 1841 pages 369, 371.) I received two dried flowers from Kew, which consisted of the two forms; in one the stigma projected far beyond the calyx, in the other it was included within the calyx; in this latter form the style was only one-fourth of the length of that in the other form. There are only six stamens; these are somewhat graduated in length, and their anthers in the short-styled form stand a little above the stigma, but yet by no means equal in length the pistil of the long-styled form. In the latter the stamens are rather shorter than those in the other form. The six stamens alternate with the petals, and therefore correspond h.o.m.ologically with the longest stamens of L. salicaria and L. Graefferi.
Lythrum hyssopifolia.
This species is said by Vaucher, but I believe erroneously, to be dimorphic. I have examined dried flowers from twenty-two separate plants from various localities, sent to me by Mr. Hewett C. Watson, Professor Babington, and others.
These were all essentially alike, so that the species cannot be heterostyled.
The pistil varies somewhat in length, but when unusually long, the stamens are likewise generally long; in the bud the stamens are short; and Vaucher was perhaps thus deceived. There are from six to nine stamens, graduated in length.
The three stamens, which vary in being either present or absent, correspond with the six shorter stamens of L. salicaria and with the six which are always absent in L. thymifolia. The stigma is included within the calyx, and stands in the midst of the anthers, and would generally be fertilised by them; but as the stigma and anthers are upturned, and as, according to Vaucher, there is a pa.s.sage left in the upper side of the flower to the nectary, there can hardly be a doubt that the flowers are visited by insects, and would occasionally be cross-fertilised by them, as surely as the flowers of the short-styled L.
salicaria, the pistil of which and the corresponding stamens in the other two forms closely resemble those of L. hyssopifolia. According to Vaucher and Lecoq, this species, which is an annual, generally grows almost solitarily (4/9.
"Geograph. Bot. de l"Europe" tome 6 1857 page 157.), whereas the three preceding species are social; and this fact alone would almost have convinced me that L.
hyssopifolia was not heterostyled, as such plants cannot habitually live isolated any better than one s.e.x of a dioecious species.
We thus see that within this genus some species are heterostyled and trimorphic; one apparently heterostyled and dimorphic, and one h.o.m.ostyled.
Nesaea verticillata.
I raised a number of plants from seed sent me by Professor Asa Gray, and they presented three forms. These differed from one another in the proportional lengths of their organs of fructification and in all respects, in very nearly the same way as the three forms of Lythrum Graefferi. The green pollen-grains from the longest stamens, measured along their longer axis and not distended with water, were 13/7000 of an inch in length; those from the mid-length stamens 9 to 10/7000, and those from the shortest stamens 8 to 9/7000 of an inch. So that the largest pollen-grains are to the smallest in diameter as 100 to 65.
This plant inhabits swampy ground in the United States. According to Fritz Muller, a species of this genus in St. Catharina, in Southern Brazil, is h.o.m.ostyled. (4/10. "Botanische Zeitung" 1868 page 112.)
Lagerstroemia Indica.
This plant, a member of the Lythraceae, may perhaps be heterostyled, or may formerly have been so. It is remarkable from the extreme variability of its stamens. On a plant, growing in my hothouse, the flowers included from nineteen to twenty-nine short stamens with yellow pollen, which correspond in position with the shortest stamens of Lythrum; and from one to five (the latter number being the commonest) very long stamens, with thick flesh-coloured filaments and green pollen, corresponding in position with the longest stamens of Lythrum. In one flower, two of the long stamens produced green, while a third produced yellow pollen, although the filaments of all three were thick and flesh- coloured. In an anther of another flower, one cell contained green and the other yellow pollen. The green and yellow pollen-grains from the stamens of different length are of the same size. The pistil is a little bowed upwards, with the stigma seated between the anthers of the short and long stamens, so that this plant was mid-styled. Eight flowers were fertilised with green pollen, and six with yellow pollen, but not one set fruit. This latter fact by no means proves that the plant is heterostyled, as it may belong to the cla.s.s of self-sterile species. Another plant growing in the Botanic Gardens at Calcutta, as Mr. J.
Scott informs me, was long-styled, and it was equally sterile with its own pollen; whilst a long-styled plant of L. reginae, though growing by itself, produced fruit. I examined dried flowers from two plants of L. parviflora, both of which were long-styled, and they differed from L. Indica in having eight long stamens with thick filaments, and a crowd of shorter stamens. Thus the evidence whether L. Indica is heterostyled is curiously conflicting: the unequal number of the short and long stamens, their extreme variability, and especially the fact of their pollen-grains not differing in size, are strongly opposed to this belief; on the other hand, the difference in length of the pistils in two of the plants, their sterility with their own pollen, and the difference in length and structure of the two sets of stamens in the same flower, and in the colour of their pollen, favour the belief. We know that when plants of any kind revert to a former condition, they are apt to be highly variable, and the two halves of the same organ sometimes differ much, as in the case of the above-described anther of the Lagerstroemia; we may therefore suspect that this species was once heterostyled, and that it still retains traces of its former state, together with a tendency to revert more completely to it. It deserves notice, as bearing on the nature of Lagerstroemia, that in Lythrum hyssopifolia, which is a h.o.m.ostyled species, some of the shorter stamens vary in being either present or absent; and that these same stamens are altogether absent in L. thymifolia. In another genus of the Lythraceae, namely Cuphea, three species raised by me from seed certainly were h.o.m.ostyled; nevertheless their stamens consisted of two sets, differing in length and in the colour and thickness of their filaments, but not in the size or colour of their pollen-grains; so that they thus far resembled the stamens of Lagerstroemia. I found that Cuphea purpurea was highly fertile with its own pollen when artificially aided, but sterile when insects were excluded. (4/11. Mr. Spence informs me that in several species of the genus Mollia (Tiliaceae) which he collected in South America, the stamens of the five outer cohorts have purplish filaments and green pollen, whilst the stamens of the five inner cohorts have yellow pollen. He therefore suspected that these species might prove to be heterostyled and trimorphic: but he did not notice the length of the pistils. In the allied Luhea the outer purplish stamens are dest.i.tute of anthers. I procured some specimens of Mollia lepidota and speciosa from Kew, but could not make out that their pistils differed in length in different plants; and in all those which I examined the stigma stood close beneath the uppermost anthers. The numerous stamens are graduated in length, and the pollen-grains from the longest and shortest ones did not present any marked difference in diameter. Therefore these species do not appear to be heterostyled.)]
Oxalis (Geraniaceae).
(Figure 4.11. Oxalis speciosa (with the petals removed).
Left: Long-styled.
Centre: Mid-styled.
Right: Short-styled.
S, S, S, stigmas. The dotted lines with arrows show which pollen must be carried to the stigmas for legitimate fertilisation.)
In 1863 Mr. Roland Trimen wrote to me from the Cape of Good Hope that he had there found species of Oxalis which presented three forms; and of these he enclosed drawings and dried specimens. Of one species he collected 43 flowers from distinct plants, and they consisted of 10 long-styled, 12 mid-styled, and 21 short-styled. Of another species he collected 13 flowers, consisting of 3 long-styled, 7 mid-styled, and 3 short-styled. In 1866 Professor Hildebrand proved by an examination of the specimens in several herbaria that 20 species are certainly heterostyled and trimorphic, and 51 others almost certainly so.
(4/12. "Monatsber. der Akad. der Wiss. Berlin" 1866 pages 352, 372. He gives drawings of the three forms at page 42 of his "Geschlechter-Vertheilung" etc.
1867.) He also made some interesting observations on living plants belonging to one form alone; for at that time he did not possess the three forms of any living species. During the years 1864 to 1868 I occasionally experimented on Oxalis speciosa, but until now have never found time to publish the results. In 1871 Hildebrand published an admirable paper in which he shows in the case of two species of Oxalis, that the s.e.xual relations of the three forms are nearly the same as in Lythrum salicaria. (4/13. "Botanische Zeitung" 1871 pages 416 and 432.) I will now give an abstract of his observations, and afterwards of my own less complete ones. I may premise that in all the species seen by me, the stigmas of the five straight pistils of the long-styled form stand on a level with the anthers of the longest stamens in the two other forms. In the mid- styled form, the stigmas pa.s.s out between the filaments of the longest stamens (as in the short-styled form of Linum); and they stand rather nearer to the upper anthers than to the lower ones. In the short-styled form, the stigmas also pa.s.s out between the filaments nearly on a level with the tips of the sepals.
The anthers in this latter form and in the mid-styled rise to the same height as the corresponding stigmas in the other two forms.
Oxalis Valdiviana.
This species, an inhabitant of the west coast of South America, bears yellow flowers. Hildebrand states that the stigmas of the three forms do not differ in any marked manner, but that the pistil of the short-styled form alone is dest.i.tute of hairs. The diameters of the pollen-grains are as follows:--
Table 4.b. Oxalis Valdiviana. Diameters of pollen-grains in divisions of the micrometer.
Column 1: Source of Pollen-grains.
Column 2: Minimum diameter.
Column 3: Maximum diameter.
From the: Longest stamens of short-styled form : 8 to 9.
Mid-length stamens of short-styled form : 7 to 8.
Longest stamens of mid-styled form : 8.
Shortest stamens of mid-styled form : 8.
Mid-length stamens of long-styled form : 7.
Shortest stamens of long-styled form : 6.
Therefore the extreme difference in diameter is as 8.5 to 6, or as 100 to 71.
The results of Hildebrand"s experiments are given in Table 4.27, drawn up in accordance with my usual plan.
Table 4.27. Oxalis Valdiviana (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union : 28 : 28 : 11.9.