have become regularly autogamous, while others are now anemophilous, it may be due to the fact that, if a flower has been entomophilous and even strongly protandrous, the first stage of degradation is to bring the essential organs to a homogamous state. If they stop there, and become autogamous as well, which is the usual result, then the flower will remain persistently self-fertilising, as, e.g., Shepherd's-purse, Chickweed, Knot-grass, etc.

If, however, the flower had been protogynous, such as early-flowering Hellebores, Prunus communis or some Alpine species, with "long-lived stigmas," then this protogyny, associated with other degradations of the corolla, etc., which only tend to increase it, has ended with anemophily.

In the first case the androecium of protandrous flowers has come down from its previous highly differentiated state, so as to be homogamous with the stigmas. From the other or protogynous condition, the gynoecium has not been brought back again so as to be homogamous with the anthers and pollen, but, on the contrary, it may have become even further differentiated, and so has now no fertiliser to depend upon except the wind.

CHAPTER XXVII.

DEGENERACY OF FLOWERS (continued).

DEGENERACY OF THE ANDRŒCIUM.-The number of stamens may decrease, as well as the quantity of pollen; while the form of the anthers may change and the character of the pollen may alter; and lastly, the position of the stamens may not be the same as in intercrossing flowers,—all these forms of degradation being so many adaptations or adjustments for self-fertilisation. They are well seen in Violets and the

Wood-sorrel.

As examples, in Stellaria Holostea there are ten stamens, in S. media only three; and in cleistogamous Violets they vary from five to three or two. In the latter, the anthers become spoon-shaped with a rounded connective and much reduced anther cells; in the cleistogamous flowers of Oxalis Acetosella the pollen is almost deliquescent. Lastly, in all flowers especially adapted for self-fertilisation the anthers are in contact with the stigmas in consequence of their arrest in growth.

It must be noted here that this degeneracy in the stamens in no way impairs their functional value. The fact is that a very small amount of pollen is really quite sufficient for fertilising a considerable number of ovules.

For convenience I call it degeneracy, but another view would be to regard it as the conservation of energy, instead of

T

wasting it in the production of a great deal more pollen than is usually required.

*

An interesting experiment of Mr. Darwin's proves this. He placed a very small mass of pollen-grains on one side of the large stigma of Ipomoea purpurea, and a great mass of pollen over the whole surface of the stigmas of other flowers, and the result was that the flowers fertilised with little pollen yielded rather more capsules and seeds than did those fertilised with an excess. That normally intercrossing flowers produce a great superfluity of pollen is well known. Thus Kölreuter found that sixty grains were necessary to fertilise all the ovules of a flower of Hibiscus, while he calculated that 4863 grains were produced by a single flower, or eighty-one times too many. Mr. Darwin says, "In order to compensate the loss of pollen in so many ways, the anthers produce a far larger amount than is necessary for the fertilisation of the same flower; . . . and it is still more plainly shown by the astonishingly small quantity produced by cleistogene flowers, which lose none of their pollen, in comparison with that produced by the open flowers borne by the same plants; and yet this small quantity suffices for the fertilisation of all their numerous seeds."

Mr. Darwin observed that when flowers were artificially self-fertilised for several successive generations, a degeneracy sometimes took place in the anthers and pollen; and he seems to attribute this to what he called the "evil effects" of selffertilisation; but from the above-mentioned facts, which occur so abundantly in nature, I am inclined to regard it as an experimental verification and illustration of a universal principle in nature, namely the preservation of energy wherever possible, and that such cases as appeared under his *Cross and Self Fertilisation of Plants, p. 25. + Ibid., pp. 376, 377.

experiments were instances of this principle at work, as the flowers became habituated to self-fertilisation, and were then fully fertile.

We have, then, in such cases an actual demonstration of the first step of the changes induced by self-fertilisation continually enforced; and thereby a witness to one cause of the origin of certain, and indeed, a very large number of species. It is the converse process to that of insect fertilisation, which itself I take to be the vera causa of the origin of intercrossing species.

It is, perhaps, worthy of note that, while both the number of stamens and the quantity of pollen are thus often much reduced in some flowers the capsules of which produce many seeds, yet in others which set but one, as Fumaria, or at least but few seeds, the number of stamens may remain unaltered. This seems to me to be an additional proof that such flowers are degradations from forms originally adapted to intercrossing when much more pollen was requisite. Hence the present forms are retentions of former ancestral conditions. The following cases will illustrate this :Scleranthus perennis and species of Medicago have ten stamens and one seed; Daphne Laureola has eight stamens and one seed; Chenopodium has five stamens and one seed; similarly is it the case with the large orders Composite and Gramineæ. The phenomenon called "contabescence contabescence" by Gärtner would seem to have its rationale in this adaptation to selffertilisation in some cases, and to diclinism in others, though there are other causes which may bring it about, when it is a purely pathological phenomenon.

*

Mr. Darwin observes, "The anthers are affected at a very early period in the flower-bud, and remain in the same state (with one recorded exception) during the life of the

* An. and Pl. under Dom., ii., p. 165.

plant. The affection cannot be cured by any change of treatment, and is propagated by layers, cuttings, etc., and perhaps even by seed. In contabescent plants the female organs are seldom affected, or merely become precocious in their development. The cause of this affection is doubtful, and is different in different cases. . . The contabescent plants of Dianthus and Verbascum found wild by Wiegmann grew on a dry and sterile bank.” *

"Cases of an opposite nature likewise occur namely, plants with the female organs struck with sterility, whilst the male organs remain perfect."

The constancy or prevalence of this condition of contabescence seems to be the first indication of diclinism, whatever the cause; and Silene inflata may be mentioned as frequently furnishing good examples of both kinds of

contabescence.

DEGENERACY OF THE POLLEN.-As this is a feature of importance in the general degradation of flowers, a few words may be added in reference to it. It is of frequent occurrence in cultivated plants; thus Potatoes are notorious for failing to produce fruit; and some varieties are much less liable to do so than others. Mr. C. F. White, F.L.S., tells me he regards this plant as furnishing the most conspicuous example of a form of degradation of pollen; the pollen grains of a normal character are very generally not to be found at all, but round, square, and polygonal forms abound. On the other hand, he gathered many flowers, in a large field in the Isle of Thanet, with scarcely a grain imperfect in shape or reduced in size.

Mr. White has noticed, in his numerous researches among pollens, that degeneracy by dwarfing is mostly or very frequently induced by inclement weather. He mentions * A like cause produces petalody of stamens, see p. 299.