ANDRODIŒCISM AND ANDROMONŒCISM.

-These conditions

do not appear to prevail to the same extent as the female forms of flowers. Both of these kinds are not at all uncommon in the Umbellifera, and are a result of exhaustion, for the umbels produced at the end of the season are often entirely male; or, if at other periods, it is generally the central florets which develop no pistils, as in Astrantia minor. Müller has noticed how "the weaker plants usually bear but one umbel consisting only of male flowers." This would make it androdioecious. I find that andromonoecism prevails in Astrantia major, Carum, Smyrnium, and in Trinia vulgaris. This last, growing on the Clifton downs, bore umbels which were altogether male, after the hermaphrodite ones had formed their fruit. Daucus grandiflora is remarkable for having three kinds of flowers. According to Müller, the central ones are male; at the edge of the umbellule the flowers are neuter, with the outermost petal greatly enlarged; lastly, at the margin of the whole umbel, are female florets in which the outer petals attain to a gigantic size.†

* Androdiacism signifies that the same species has both male and hermaphrodite plants.

Andromonacism signifies that the same plant bears both male and hermaphrodite flowers.

† I would here remind the reader that the interpretation given above (Chapters XI.-XIII.) of the origin of irregular corollas, applies equally well to those cases where it is only in the outermost florets of a cluster where the petals are enlarged, as in Iberis, many of the Composite, and Umbelliferæ, as well as in Hydrangea, Guelder Rose, etc. In all these, when insects first approach the umbel and alight on the border of it, any or each individual floret on the margin may have to carry the burden. As soon, however, as the insect passes the edge of the cluster, its weight is distributed over several florets; so that they are not submitted to any special strains upon one, i.e. the outer side only. The same remarks apply to Mentha, as compared with Lamium. The insect visits one flower at a time in the latter, but scrambles over several in the former, which has (presumably) degraded in consequence.

Caltha palustris is said to be androdioecious, but no details are given by the observer.*

Besides the Umbellifera,† where andromonoecism seems to be a characteristic feature, Müller mentions Asperula taurina and Galium Cruciata, Pulmonaria officinalis, Coriaria myrtifolia, and Diospyrus Virginiana as being andromonoecious. The hermaphrodite flowers of these species are protandrous.

In Galium Cruciata, Mr. Darwin noticed that the pistil is suppressed in most of the lower flowers, the upper remaining hermaphrodite.

Heterostylism may tend to produce the same result when the stamens of the long-styled forms degenerate so far as to become atrophied without the pistil losing its functions. Pulmonaria angustifolia and Phlox subulata give hints of this condition. Asperula scoparia was at first thought by Mr. Darwin to be heterostyled, but finding the anthers to be destitute of pollen, he considered it to be dioecious. A. taurina, as figured by Müller,§ shows great variability in the lengths of the filaments and styles, and he pronounces it to be andromonoecious. Hence, as so many of the Rubiacea are heterostyled, there seems every probability of one result of this peculiarity, being one or other kind of this incompletely affected or partial diclinism. In the case of Coriaria myrtifolia, Hildebrand found that it was the first flowers which were male only. In Maples, as in Galium Cruciata, the rule is for the three or more flowered corymb to have the central one hermaphrodite, and the lower or outer ones male. This *Lecoq, Geóg. Bot., tom. iv., p. 488.

† Müller says that in Sanicula Europea the outer flowers are male, and develop after the inner ones, which are hermaphrodite. This is so anomalous, that one suspects an error somewhere. I have not had any opportunity of examining fresh flowers.

Forms of Flowers, p. 287.

§ Fertilisation, etc., p. 303.

clearly is a question of the distribution of nutrition; the lower, being the later ones to expand, are the weaker.* Müller mentions Horse-chestnuts as being also andromonoecious; and what is exceptional is that the hermaphrodite flowers are protogynous. This, however, may be due to the early period of flowering, like species of Prunus and Crataegus.

The reader will now perceive that there may be several causes at work to produce these kinds of "partial diclinism;" and that what is required is to ascertain, if possible, by observation and experiment, which is the one peculiar to each species. Secondly, when any one or more causes has been sufficiently persistent, the results become hereditary; so that certain species, genera, and orders become more or less characterized by these peculiar features.

* Compare the observations on Adoxa, p. 188.

CHAPTER XXV.

SEXUALITY AND THE ENVIRONMENT.

GENERAL OBSERVATIONS.-As the environment is now known to have most potent influences on the anatomical structure of the vegetative system of plants, thereby affecting their outward and visible morphological characters as well; so are there many causes which affect the reproductive system, at one time influencing the androecium, at another the gynocium, favouring them or the reverse as the case may be; so that either sex or even both may be entirely suppressed, and a hermaphrodite flower become male, female, or neuter.

With regard to the most general agency, there seems to be a tolerably uniform consensus of opinion that the female sex in plants is correlated with a relatively stronger vital vigour than the male; and this is just what an à priori assumption would look for, as the duration of existence and the work to be done in making fruit require a greater expenditure of energy than the temporary function of the

stamens.

We must, however, distinguish between a healthy vital vigour, and any excessive vegetative growth, as occurs under high cultivation, and as is often the result of intercrossing. If this latter surpass the requisite or optimum conditions for the healthy performance of the functions of all the organs

of a plant, then either of the sexual organs may begin to deterioriate, till they become metamorphosed into petals or leaves, or else degenerate and vanish.

66

It is true enough that we know nothing of the real nature of life; but it is easy to see that, of the various phases of development, from germination to the production of seed, each should have the proper amount of energy at its disposal, and no more; for if any one organ be stimulated beyond the optimum degree, others suffer through atrophy. The first and well-known distinction to be noticed lies, of course, between the vegetative energy," by means of which roots, stems, branches, and foliage are developed, and the "reproductive energy," which brings about the formation of flowers, fruit, and seed. If either of these be unduly excited, the other diminishes. Thus, as long as fruit trees are developing much wood and foliage, they either bear fruit badly or not at all. Plants which are propagated largely by vegetative means of multiplication, such as bulbs, corms, tubers, etc., are notorious for failing to set seed as well. As an instance in nature, Ranunculus Ficaria may be mentioned. This plant propagates itself by "root-tubers" and by aërial corms, and rarely produces much fruit, for the pollen often remains in an arrested state.* Conversely, if vegetative energy be checked by root and branch pruning, bark-ringing, etc., the reproductive energy is promoted, and an abundance of fruit is the reward. Similar results follow a decrease of energy through impoverishment, when enormous crops of fruit may be borne by trees, as I have seen in Portugal Laurels, when the roots had penetrated a bed of gravel and the branches became decayed.

Apart from these general considerations certain special conditions are found to favour one sex more than the other,

* See Van Tieghem on R. Ficaria, Ann. des Sci. Nat., v., sér. 5, p. 88.