(FIGURE 7.13. Rhamnus catharticus (from Caspary.) Left: Long-styled male. Right: Short-styled male.)
(FIGURE 7.14. Rhamnus catharticus. Left: Long-styled female. Right: Short-styled female.)
This plant is well known to be dioecious. My son William found the two sexes growing in about equal numbers in the Isle of Wight, and sent me specimens, together with observations on them. Each sex consists of two sub-forms. The two forms of the male differ in their pistils: in some plants it is quite small, without any distinct stigma; in others the pistil is much more developed, with the papillae on the stigmatic surfaces moderately large. The ovules in both kinds of males are in an aborted condition. On my mentioning this case to Professor Caspary, he examined several male plants in the botanic gardens at Konigsberg, where there were no females, and sent me the drawings in Figure 7.13.
In the English plants the petals are not so greatly reduced as represented in this drawing. My son observed that those males which had their pistils moderately well-developed bore slightly larger flowers, and, what is very remarkable, their pollen-grains exceeded by a little in diameter those of the males with greatly reduced pistils. This fact is opposed to the belief that the present species was once heterostyled; for in this case it might have been expected that the shorter-styled plants would have had larger pollen-grains.
In the female plants the stamens are in an extremely rudimentary condition, much more so than the pistils in the males. The pistil varies considerably in length in the female plants, so that they may be divided into two sub-forms according to the length of this organ. Both the petals and sepals are decidedly smaller in the females than in the males; and the sepals do not turn downwards, as do those of the male flowers when mature. All the flowers on the same male or same female bush, though subject to some variability, belong to the same sub-form; and as my son never experienced any difficulty in deciding under which class a plant ought to be included, he believes that the two sub-forms of the same sex do not graduate into one another. I can form no satisfactory theory how the four forms of this plant originated.
Rhamnus lanceolatus.
This plant exists in the United States, as I am informed by Professor Asa Gray, under two hermaphrodite forms. In the one, which may be called the short-styled, the flowers are sub-solitary, and include a pistil about two-thirds or only half as long as that in the other form; it has also shorter stigmas. The stamens are of equal length in the two forms; but the anthers of the short-styled contain rather less pollen, as far as I could judge from a few dried flowers. My son compared the pollen-grains from the two forms, and those from the long-styled flowers were to those from the short-styled, on an average from ten measurements, as 10 to 9 in diameter; so that the two hermaphrodite forms of this species resemble in this respect the two male forms of R. catharticus. The long-styled form is not so common as the short-styled. The latter is said by Asa Gray to be the more fruitful of the two, as might have been expected from its appearing to produce less pollen, and from the grains being of smaller size; it is therefore the more highly feminine of the two. The long-styled form produces a greater number of flowers, which are clustered together instead of being sub- solitary; they yield some fruit, but as just stated are less fruitful than the other form, so that this form appears to be the more masculine of the two. On the supposition that we have here an hermaphrodite plant becoming dioecious, there are two points deserving notice; firstly, the greater length of the pistil in the incipient male form; and we have met with a nearly similar case in the male and hermaphrodite forms of Euonymus compared with the females. Secondly, the larger size of the pollen-grains in the more masculine flowers, which perhaps may be attributed to their having retained their normal size; whilst those in the incipient female flowers have been reduced. The long-styled form of R. lanceolatus seems to correspond with the males of R. catharticus which have a longer pistil and larger pollen-grains. Light will perhaps be thrown on the nature of the forms in this genus, as soon as the power of both kinds of pollen on both stigmas is ascertained. Several other species of Rhamnus are said to be dioecious or sub-dioecious. (7/10. Lecoq ‘Geogr. Bot.’ tome 5 1856 pages 420- 26.) On the other hand, R. frangula is an ordinary hermaphrodite, for my son found a large number of bushes all bearing an equal profusion of fruit.
Epigaea repens (ERICACEAE).
This plant appears to be in nearly the same state as Rhamnus catharticus. It is described by Asa Gray as existing under four forms. (7/11. ‘American Journal of Science’ July 1876. Also ‘The American Naturalist’ 1876 page 490.) (1.) With long style, perfect stigma, and short abortive stamens. (2.) Shorter style, but with stigma equally perfect, short abortive stamens. These two female forms amounted to 20 per cent of the specimens received from one locality in Maine; but all the fruiting specimens belonged to the first form. (3.) Style long, as in Number 1, but with stigma imperfect, stamens perfect. (4.) Style shorter than in the last, stigma imperfect, stamens perfect. These two latter forms are evidently males. Therefore, as Asa Gray remarks, “the flowers may be classified into two kinds, each with two modifications; the two main kinds characterised by the nature and perfection of the stigma, along with more or less abortion of the stamens; their modifications, by the length of the style.” Mr. Meehan has described the extreme variability of the corolla and calyx in this plant, and shows that it is dioecious. (7/12. “Variations in Epigaea repens” ‘Proc. Acad. Nat. Soc. of Philadelphia’ May 1868 page 153.) It is much to be wished that the pollen-grains in the two male forms should be compared, and their fertilising power tried on the two female forms.
Ilex aquifolium (AQUIFOLIACEAE).