absolute in nature, so in this case, plants respond to the influences brought to bear upon them, and each individual may vary accordingly, but if the influence be permanent, then the variation becomes hereditary, and one or other character is fixed, and may be regarded as specific or generic as the case may be. Should the environment change again, what may have been constant for generations will be once more broken up, and instability ensues.

Müller records several cases of such oscillations, as in Pulsatilla vernalis, Dryas octopetala, Ribes petræum, Gentiana campestris, Veronica serpyllifolia, V. spicata, Walnut, Hazel, etc. These vary from protandry through homogamy to protogyny. He also mentions species which have not yet arrived at complete protogyny, such as Sibbaldia procumbens and Ranunculus alpestris, mountain species which are homogamous; while R. glacialis is sometimes even slightly protandrous. Papaver alpinum, Arabis alpina, and Biscutella laevigata are also described as homogamous.

As the transitions from a conspicuous, protandrous, and entomophilous or insect-fertilised flower to a homogamous and autogamous or self-fertilised one, as well as to anemophily, are the effects of degeneracy, they will be considered more fully when that peculiar condition of floral structure comes to be discussed.*

*

* See Chaps. XXVI. and XXVII.

CHAPTER XXIII.

HETEROSTYLISM.*

DIMORPHIC FLOWERS.-A large portion of Mr. Darwin's work on the "Forms of Flowers" deals with the varieties and phenomena of heterostylism, which is specially characteristic of the Primulaceae, and Rubiaceae, though several instances exist in other orders as well. He and Mr. J. Scott were mainly interested in showing that "illegitimate" or homomorphic unions were less prolific than "legitimate" or heteromorphic; and inferentially took occasion to describe the differential sexual characters of the forms of the same species. With regard to this latter fact, when Mr. Darwin experimented with wild Cowslips, he first thought that they were tending towards a dioecious condition, and that the long-styled plants were more feminine in nature, and would produce more seed : conversely, that the short-styled plants were more masculine.

Contrary to his anticipation, of plants marked growing in his garden, in an open field, and in a shady wood, the short-styled forms gave most seed, the weight of seed being in the proportion of 41 to 34; that is, the short-styled produced more seed than the long-styled in the proportion of nearly 4 to 3. Similarly when a number of wild plants were *Heterostyled, i.e. plants with stamens and styles of different but corresponding lengths on separate plants.

Homostyled, i.e. when stamens and styles are of the same length. Homo-, di-, tri-, poly-, and hetero-morphic, i.e. flowers of the same, two, three, many, and different forms, respectively.

transferred to his garden, the result was as 430 to 332, the weight of seed being therefore nearly 4 to 3. Lastly, of plants covered by a net, six short-styled plants bore about 50 seeds, while 18 long-styled plants bore none at all.

From these results. Mr. Darwin wrote, "we may safely conclude that the short-styled form is more productive than the long-styled form. . . . Consequently my anticipation that the long-styled form would prove to be more feminine in nature, is exactly the reverse of the truth.”* We shall see, however, that his surmise was probably, to some extent, right, nevertheless.

Mr. Darwin and Mr. Scott have recorded a great number of experiments in crossing heterostyled plants, and the following tables, constructed from details given by those authors, show to what extent the plants named were benefited by crossing either way.

LEGITIMATE OR HETEROMORPHIC UNIONS.

Primula veris (Wt. of seeds of 100 capsules) +2

The first observation is that in twelve cases the shortstyled are in excess of the long-styled, and in four cases (†) this is reversed. Hence Mr. Darwin's conclusion is not absolute; and it is a somewhat remarkable fact that Primula veris (the Cowslip) is the identical species from which he deduced the conclusion that the short-styled was the more feminine of the two forms. The conclusion now arrived at from this species would be, that when it is left to itself the short-styled form sets most seed; but when artificially crossed it is the long-styled form which bears best. The cause of the former result is that some pollen in the short-styled form can fall upon the stigma and so secure self-fertilisation, which is impossible in the latter case. The same results occurred with Mr. Scott.*

Hence Mr. Darwin's first conclusion, that the short-styled was the more feminine, was drawn from a wrong premise; as it was not a question of sex so much as of union. When the results of self-fertilisation are compared, as given in the table on next page, it appears that the long-styled form of the Cowslip is the more feminine of the two, in the proportion of 42 to 30.

Of that table, three cases of Primula sp. (†) only show the short-styled bearing more seed than the long-styled when illegitimately fertilised; viz., with Mr. Scott, P. vulgaris, var. alba, and P. Auricula (i.e. forms more or less modified by cultivation); and with Hildebrand, P. Sinensis, when crossed

*Journ. Linn. Soc. Bot., vol. viii., 1864. This case may be taken to illustrate one of the disadvantages often accruing through great differentiation and adaptation to insect visitors. Though it appears proved that legitimate crossing sets most seed when carefully and artificially effected; yet, when the process is left to the capricious visits of insects, Mr. Darwin's experiments show how nature fails to derive the full benefit of intercrossing; so that the Cowslip has to be contented with the results of the illegitimate union of the least fertile of the two forms.

by distinct plants. The difference, however, being only two in each ease, is practically inappreciable.

Of the other genera, Linum shows a slight inclination in favour of short-styled; but as this genus is exceedingly barren when illegitimately fertilised, the results here given of that plant are insufficient for deducing conclusions; at all events, these tables show that the long-styled form is certainly more prolific when illegitimately fertilised, than the short-styled form when similarly treated.*

ILLEGITIMATE OR HOMOMORPHIC UNIONS.

Referring to the column of Differences in the first table, it will be noticed that two of the four marked (†) of the longstyled are considerable, namely, P. veris and Hottonia; but the

* Mr. Darwin noticed that this was the case with the genus Primula (l.c., p. 48).