In the several works which I have consulted, one author alone says that the holly is dioecious. (7/13. Vaucher ‘Hist. Phys. des Plantes d’Europe’ 1841 tome 2 page 11.) During several years I have examined many plants, but have never found one that was really hermaphrodite. I mention this genus because the stamens in the female flowers, although quite destitute of pollen, are but slightly and sometimes not at all shorter than the perfect stamens in the male flowers. In the latter the ovary is small and the pistil is almost aborted. The filaments of the perfect stamens adhere for a greater length to the petals than in the female flowers. The corolla of the latter is rather smaller than that of the male. The male trees produce a greater number of flowers than the females. Asa Gray informs me that I. opaca, which represents in the United States our common holly, appears (judging from dried flowers) to be in a similar state; and so it is, according to Vaucher, with several other but not with all the species of the genus.
GYNO-DIOECIOUS PLANTS.
The plants hitherto described either show a tendency to become dioecious, or apparently have become so within a recent period. But the species now to be considered consist of hermaphrodites and females without males, and rarely show any tendency to be dioecious, as far as can be judged from their present condition and from the absence of species having separated sexes within the same groups. Species belonging to the present class, which I have called gyno- dioecious, are found in various widely distinct families; but are much more common in the Labiatae (as has long been noticed by botanists) than in any other group. Such cases have been noticed by myself in Thymus serpyllum and vulgaris, Satureia hortensis, Origanum vulgare, and Mentha hirsuta; and by others in Nepeta glechoma, Mentha vulgaris and aquatica, and Prunella vulgaris. In these two latter species the female form, according to H. Muller, is infrequent. To these must be added Dracocephalum Moldavicum, Melissa officinalis and clinopodium, and Hyssopus officinalis. (7/14. H. Muller ‘Die Befruchtung der Blumen’ 1873 and ‘Nature’ 1873 page 161. Vaucher ‘Plantes d’Europe’ tome 3 page 611. For Dracocephalum Schimper as quoted by Braun ‘Annals and Magazine of Natural History’ 2nd series volume 18 1856 page 380. Lecoq ‘Geographie Bot. de l’Europe’ tome 8 pages 33, 38, 44, etc. Both Vaucher and Lecoq were mistaken in thinking that several of the plants named in the text are dioecious. They appear to have assumed that the hermaphrodite form was a male; perhaps they were deceived by the pistil not becoming fully developed and of proper length until some time after the anthers have dehisced.) In the two last-named plants the female form likewise appears to be rare, for I raised many seedlings of both, and all were hermaphrodites. It has already been remarked in the Introduction that andro-dioecious species, as they may be called, or those which consist of hermaphrodites and males, are extremely rare, or hardly exist.
Thymus serpyllum.
The hermaphrodite plants present nothing particular in the state of their reproductive organs; and so it is in all the following cases. The females of the present species produce rather fewer flowers and have somewhat smaller corollas than the hermaphrodites; so that near Torquay, where this plant abounds, I could, after a little practice, distinguish the two forms whilst walking quickly past them. According to Vaucher, the smaller size of the corolla is common to the females of most or all of the above-mentioned Labiatae. The pistil of the female, though somewhat variable in length, is generally shorter, with the margins of the stigma broader and formed of more lax tissue, than that of the hermaphrodite. The stamens in the female vary excessively in length; they are generally enclosed within the tube of the corolla, and their anthers do not contain any sound pollen; but after long search I found a single plant with the stamens moderately exserted, and their anthers contained a very few full-sized grains, together with a multitude of minute empty ones. In some females the stamens are extremely short, and their minute anthers, though divided into the two normal cells or loculi, contained not a trace of pollen: in others again the anthers did not exceed in diameter the filaments which supported them, and were not divided into two loculi. Judging from what I have myself seen and from the descriptions of others, all the plants in Britain, Germany, and near Mentone, are in the state just described; and I have never found a single flower with an aborted pistil. It is, therefore, remarkable that, according to Delpino, this plant near Florence is generally trimorphic, consisting of males with aborted pistils, females with aborted stamens, and hermaphrodites. (7/15. ‘Sull’ Opera, la Distribuzione dei Sessi nelle Piante, etc’ 1867 page 7. With respect to Germany H. Muller ‘Die Befruchtung etc.’ page 327.)
I found it very difficult to judge of the proportional number of the two forms at Torquay. They often grow mingled together, but with large patches consisting of one form alone. At first I thought that the two were nearly equal in number; but on examining every plant which grew close to the edge of a little overhanging dry cliff, about 200 yards in length, I found only 12 females; all the rest, some hundreds in number, being hermaphrodites. Again, on an extensive gently sloping bank, which was so thickly covered with this plant that, viewed from the distance of half a mile it appeared of a pink colour, I could not discover a single female. Therefore the hermaphrodites must greatly exceed in number the females, at least in the localities examined by me. A very dry station apparently favours the presence of the female form. With some of the other above-named Labiatae the nature of the soil or climate likewise seems to determine the presence of one or both forms; thus with Nepeta glechoma, Mr. Hart found in 1873 that all the plants which he examined near Kilkenny in Ireland were females; whilst all near Bath were hermaphrodites, and near Hertford both forms were present, but with a preponderance of hermaphrodites. (7/16. ‘Nature’ June 1873 page 162.) It would, however, be a mistake to suppose that the nature of the conditions determines the form independently of inheritance; for I sowed in the same small bed seeds of T. serpyllum, gathered at Torquay from the female alone, and these produced an abundance of both forms. There is every reason to believe, from large patches consisting of the same form, that the same individual plant, however much it may spread, always retains the same form. In two distant gardens I found masses of the lemon-thyme (T. citriodorus, a var. of T. serpyllum, which I was informed had grown there during many years, and every flower was female.
With respect to the fertility of the two forms, I marked at Torquay a large hermaphrodite and a large female plant of nearly equal sizes, and when the seeds were ripe I gathered all the heads. The two heaps were of very nearly equal bulk; but the heads from the female plant numbered 160, and their seeds weighed 8.7 grains; whilst those from the hermaphrodite plant numbered 200, and their seeds weighed only 4.9 grains; so that the seeds from the female plant were to those from the hermaphrodite as 100 to 56 in weight. If the relative weight of the seeds from an equal number of flower-heads from the two forms be compared, the ratio is as 100 for the female to 45 for the hermaphrodite form.
Thymus vulgaris.
(FIGURE 7.15. Thymus vulgaris (magnified). Left: Hermaphrodite. Right: Two females.)
The common garden thyme resembles in almost every respect T. serpyllum. The same slight differences between the stigmas of the two forms could be perceived. In the females the stamens are not generally quite so much reduced as in the same form of T. serpyllum. In some specimens sent me from Mentone by Mr. Moggridge, together with the sketches in Figure 7.15, the anthers of the female, though small, were well formed, but they contained very little pollen, and not a single sound grain could be detected. Eighteen seedlings were raised from purchased seed, sown in the same small bed; and these consisted of seven hermaphrodites and eleven females. They were left freely exposed to the visits of bees, and no doubt every female flower was fertilised; for on placing under the microscope a large number of stigmas from female plants, not one could be found to which pollen-grains of thyme did not adhere. The seeds were carefully collected from the eleven female plants, and they weighed 98.7 grains; and those from the seven hermaphrodites 36.5 grains. This gives for an equal number of plants the ratio of 100 to 58; and we here see, as in the last case, how much more fertile the females are than the hermaphrodites. These two lots of seeds were sown separately in two adjoining beds, and the seedlings from both the hermaphrodite and female parent-plants consisted of both forms.