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PARASITISM IN FLOWERING PLANTS.

BY HENRY TRIMEN, M.B., F.L.S.
LECTURER ON BOTANY, ST. MARY'S HOSPITAL.

[PLATES XCIX. and C.]

THE tendency of words to degenerate in meaning has frequently been commented on by philologists, and to the more commonly known examples may be added the term 'parasite.' Originally, in early Greek times, applied to the members of a college of priests (Tapáσirol) it possessed simply its grammatical meaning of taking meals in common, but later it came to signify living at another's expense, and was then applied to the contemptible toady and flatterer-one of the stock characters in Greek and Roman comedy-who would put up with any indignity from his patron for the sake of food and lodging. This character, which the social habits of classical times seem to have brought to great perfection, is by no means extinct in our days, though the objects sought by such voluntary degradation may be generally higher than mere creature enjoyments. The word is now however rather rarely applied to human beings, nor are we here concerned with such a use of it. In science also, the term parasite is given to beings which live upon others in the sense of getting from them their food, and is applied to animals which batten upon others, and to plants which are attached to, and live upon the juices, either of animals or of other vegetables.

Such habits, inimical to other creatures, are possessed by a very large number of plants. Probably all the Fungi are parasites, and a good many attack the lower animals, among insects frequently causing their death; whilst even the body of man himself is the suitable soil for about a score of minute species. Of these lowly organised plants, it is not however intended to speak, and indeed their mode of parasitism is very different from that of the flowering plants which follow that mode of life. Among the Phanerogams, we find that there are only five or six natural orders all the members of which are parasitic; but there are isolated genera, or species scattered through about a

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dozen other families. A list, which might probably be somewhat enlarged, is given below, upon which it may be remarked that Monocotyledons are few, and that none of the Dicotyledons are polypetalous (unless it be preferred to consider Balanophoracea, as belonging to that division)

Dicotyledones.

[Polypetala-none.]

Monopetala:

Convolvulaces-Cuscuta.

Ericaceae-Monotropa, Pyrola aphylla, &c.

Lennoace, Solms.- all.

Pholisma.)

(Ammobroma, Lennoa, Corallophyllum,

Scrophulariaceae-Harveya, &c., Striga, &c., Rhinanthus, &c.

Orobanchacer-all.

Gentianacea-Toyria, &c.

Gesneraceæ Christisonia.

Apetalæ :

Lauracea-Cassytha.

Loranthaceae-very nearly all.

Santalacea-Thesium, &c.

Cytinaces-all.

Rafflesiace-all.

Balanophoracer-all.

Monocotyledones:

Melanthaceae-Petrosaia.

Orchidea-Neottia, &c.
Triurideæ-all?

What is called "habit," is a very fallacious guide to affinity, and has frequently proved so to even experienced systematists. It mainly results from modifications of the organs which for purposes of classification are of minor importance. Some of the parallelisms in outward appearance between plants with little real affinity have been noticed by A. W. Bennett, in an article in this Review for January 1872, and the matter is of that curious and suggestive character which leads to many speculations. The parasitic facies is a very marked one, and has naturally had an undue prominence assigned to it by some systematic botanists. Lindley and Endlicher, for example, created a class under the name of Rhizogens or Rhizantheæ to include the orders Rafflesiacea, Cytinacea and Balanophoraceae, which was considered to be intermediate between thallogenous Cryptogams (like the Fungi) and Endogens. Though these eminent authors endeavoured to support their views by various arguments drawn from supposed constant peculiarities of the embryo and the tissues, the group was no doubt founded chiefly on the habit of the plants composing it. This, however, results

from adaptive or "homoplastic" modifications fitting the plants for the peculiar conditions of their life. The last-mentioned order has but very slight affinity with the two first, and indeed the group hung but loosely together, and is now generally abandoned; all three orders are doubtless dicotyledonous with well-marked affinities to other orders of that class.

Before we examine the peculiarities in question, it will be as well to recall to the mind of the reader the usual constitution of an ordinary phanerogam. Functionally, all its organs are either nutritive or reproductive; that is, they are employed in processes which either contribute directly to the maintenance of the life of the individual to which they belong, or are directed towards the formation of new individuals in an embryo state which shall reproduce the species. The former system of plant-organs includes the green leaves spread out in the sunshine, the absorbent root-hairs buried in the soil or immersed in water-both food-obtaining organs, and both periodically renewed-and the whole system of axes, stem, branches and root, which carry these temporary structures, and are themselves of more solid and durable make, fitted to be not only supports, but also channels through which pass in various directions, according to the necessities of the plant, the sap and other juices. The structure of the widely separated leaves and roothairs is very different, and adapted in either case to the surrounding conditions. The delicate walls of the latter readily allow the passage of fluid, and through them is constantly streaming into the plant a current of water charged with carbon-dioxide, and containing, copiously diluted, the varied mineral and nitrogenous constituents of the soil needed by the plant. This fluid is then transmitted upwards from cell to cell, through the structure of the axial system, and is especially drawn to its extremities and to the leaves where growth is going on rapidly, and evaporation is great. The skin which covers these parts is, on the leaves especially, provided with minute orifices (stomata), so that the atmospheric air freely passes into the loosely-built cell-structure below. The walls of these cells are thin; the substance chiefly needed for the formation of plant-structures is carbon, and this is one constituent of carbon-dioxide, a gas which always exists in the atmosphere in small quantity as the result of combustion and animal respiration. This gas the leaf-cells extract from the air, it passes through the walls, and is reduced to its elements, almost the whole of the oxygen being returned to the air whilst the carbon is retained and assimilated. At the same time are produced, by combination of the constituents of the sap, the organic compounds, often very complex, which are proper to the plant. These very remarkable processes occur chiefly in those parts which possess the characteristic

green colour of healthy vegetation, which is due to the presence in the cells of a substance called chlorophyll. There can be little or no doubt that this is the main agent in the assimilation of carbon. It is well known that plants deprived of sunlight develope none of this green colouring matter, and that their tissues are weak and flabby;

Pale, fleshy, as if the decaying dead

With a spirit of life had been animated.

Such plants cannot take up carbon-dioxide; on the contrary, and this is also true of the parts of all plants which are not green -as the bark and the flower-they expire that gas, and so vitiate the atmosphere like animals.

By these processes the plant increases in size and complexity, and at the same time is able to store up in its tissues starch and other carbonaceous substances to be employed as fuel during the flowering and fruiting seasons. These reproductive functions do not commence till a period pretty definite in the life of the species, when, instead of unfolding green leaves, the buds produce those variously modified organs which form the flower and the end of all of which is directed to the production of ripe seed. The process of their evolution, and the changes they pass through, are all wasteful to the plant and involve the consumption of its hoarded stores: so great an effort is flowering that it frequently causes the death of the plant.

The modifications in external anatomy which are met with in parasites are chiefly in the organs of nutrition. Disregarding altogether for the present the green parasites, such as the Mistletoe and Loranthus, the first thing that strikes the observer is the absence of leaves. True there are not wanting a few fleshy or dry scales, but of green foliage, and indeed of green colour at all, the plants are absolutely devoid. Nor can we find root-hairs; indeed, in many no root at all can be said to exist, but where there is a branched root, it possesses instead of root-hairs suckers applied to the tissues of the supporting plant. Even the supporting axis may be greatly reduced, and so the whole nutritive system be wanting, as in the Rafflesiacea, where the base of the flower is in close apposition to the tissues of the alien stem upon which it grows. From this extreme case we may pass to parasites with a short axis closely covered with large scales (Cytinus), with a prostrate, half-buried rhizome, angular and branched (Hydnora) or very large and swollen (Balanophoraceae), and so reach plants with an erect, welldeveloped scale-bearing axis, as the Broomrapes (Orobanche). In another type, as Dodder (Cuscuta) and Cassytha, the axis is present in the form of long slender twining threads, upon which are placed the absorbent suckers, but scarcely a vestige of any

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