BOOKS.
INSECTIVOROUS PLANTS.*
To say that Mr. Darwin has contributed more than any living author towards the establishment of the current of scientific -thought which marks the present generation, or that he has pub- * Insec'frorotes Plants. By Charles Darwin. London Murray.
Milted as much minute observation as any naturalist of our time, is but a faint expression of the value we place on his labours. Any one of the many books he has published would be enough to establish a great reputation, and not the least noteworthy of these is the one now under notice. But when we remember that his labours date back now for nearly half a century, we can only look upon the patient and indomitable industry which gives us such a book at a time of life when most men are at least anxious for rest, as something very wonderful. He has availed himself of his son's abilities, it is true, but everywhere throughout the work the master's hand is evident, devising experiments marked by an ingenuity which is beyond praise, and anticipating the adverse suggestions of critics at almost every point. To find fault with the book is impossible, to praise it as it deserves is difficult, to find points of weakness is not easy, and to overturn its conclusions, based as they are on work of an exhaustive kind extending over fifteen years, is hopeless indeed.
To have established the fact that there are certain plants which eat insects,—that is to say, which purposively entrap, digest, and absorb them as food, may seem but a little matter ; but taken in its full bearings on the modern doctrine of evolution, its import- ance is overwhelming. Throughout the book, however, this is placed nowhere to the front. Confining himself to the mere fact, Mr. Darwin has worked out its extremest details, regardless, in the truest spirit of science, of what they might lead up to. Bearing in mind that he is himself the father of evolution, the unpre- judiced reader cannot but feel that the modesty of this book is far from being its least merit.
The first and chief part of Mr. Darwin's observations were made on the common, round-leaved English sundew, a little marsh plant, of common occurrence and wide distribution. That this plant captures insects has been known for nearly a century, but with Mr. Darwin rests the whole merit of showing for what purpose it makes the process useful. When an insect is caught on the glands placed at the ends of the long fingers which cover the upper sur- face of the leaf, these fingers bend and deliberately place their prey on the disc of the leaf, which must be regarded as its stomach. There is no mastication, it is true, but neither is there any in the great class of Edentata, of which the ant-bear is a good instance, and which feed on exactly the same kind of food as does the sun- dew. For the purpose of catching the flies, the glands are covered with a viscid secretion which seems to have some attrac- tion for them, but when a fly has been caught and swallowed the nature of that secretion changes very materially. It becomes much increased in quantity, and it becomes acid. It kills the insect and then digests it, that is to say, it dissolves out of it everything that is soluble, and then it finally absorbs at least some part of what has been dissolved. The acid which is secreted has been investigated by Professor Frankland, who finds that it is probably propionic acid ; and the peculiar ferment which takes part in the digestion has been separated by Mr. Lawson Tait from the secretion of Drosera binata, and has been found to resemble pepsin very closely, though in some particulars it differs somewhat from that important factor in human digestion.
Even so far the occurrence of real gastric digestion in plants would have been an important discovery, and to account for its usefulness Mr. Tait, and Mr. Canby before him in the case of the Dionce,a, had suggested that the leaves only prepared manure fix the roots. To prove absorption by the leaves seemed almost beyond physiological possibility. But Mr. Darwin has accomplished it beyond dispute, and in doing so he has made a contribution to microscopic research of the utmost importance. He has dis- covered a series of phenomena occurring in the cell-protoplasm of the plant, to which he has applied the term "aggregation." For the use of this term he does not, it seems to us, give very good reason, beyond that it is partly descriptive. A better might have been found, but as Mr. Darwin has begun its use, it is certain to be continued. We cannot find space here to describe these phenomena, and indeed they would be beyond lay readers ; but we cannot dismiss them without saying that their meaning seems to have been completely elucidated by Mr. Darwin, and that his discovery opens the way to a whole series of new facts in cell- physiology.
The absorption of the digested food, therefore, by the glands of the sundew must be accepted as a fact. But still there are certain conditions of the plant which Mr. Darwin has not attached enough weight to, and which seem to support Mr. Tait's view, that the digestion may prove useful by applying food to the ab- sorbent roots. The plant may be seen growing in exposed situations, as near the gap of Dunloe, where showers of rain are frequent, so that in spring, at least, scarcely a day may pass without a leaf being visited by a rain-drop or a dew-drop The digestion of a fly takes many days, from six to ten, to accomplish, so that in the great majority of instances it is probable that some of the products of digestion are washed out of the folded disc of the leaf. When the plant is most actively growing and the whole process most energetic, the leaf is not only folded over its prey at the margins, but the wrist of the leaf is bent towards the axis of the plant, so that if a rain-drop should wash the products of digestion out of the leaf, it will do so directly on to the ground or moss near the root. The plant grows in closely- matted patches, so that an isolated plant is seldom met with, and in many cases a number of plants are so closely mixed up that it is difficult to separate them. The drops from the leaves of one plant would, therefore, be of material service to the roots of all those associated with it. Finally, plants placed in silver-sand, and so guarded that nothing can reach their leaves, flourish well when nitrogenous matter is supplied to their roots. Dr. Carpenter has established this also in the case of rye-grass, and indeed it is, of course, the normal condition of all vegetation. Even in the case of Dionoea, where the stomach closes completely, the exuberant - products of digestion may sometimes be seen trickling down a channel in the petiole towards the roots. There can be no question but that this is a provision against waste, and that the surplus food thus utilised must prove of great service to the plant. Of the direct primary absorption by the leaf there can, however, be no doubt.
Very many of the peculiarities of action of the digestive juice of the sundew on certain substances, as gelatin, chondrin, casein, fibrin, are identical with the actions of our own gastric juice ; in- deed, it is far more difficult to point to dissimilarities than to resemblances.
A few words must now be said about the facts discovered by Mr. Darwin, while investigating the movements of the leaves. Here some of the resemblances in the processes to those witnessed in animals are positively startling. These movements are most perfectly seen in the trap of the Dionce,a, and Dr. Burdon Sanderson's remarkable researches show that they are subject to exactly the same laws as govern muscular movements and nerve influence. The closure of the trap of the Dioncea must be regarded as homologous to the act of swallowing, and like that it is purely reflex. Thus after the bolus of food has reached the region of the glosso-pharyngeal nerve in our own tongues, it is beyond our control, and the reflex mechanism closes it within the stomach. When the food of the fly-trap touches one of the nerve filaments, the lobes close by a reflex movement identical in its phenomena with that seen in the animal. Mr. Darwin seems to object to the application of the term " nerve " to the filaments of the Dioncea, but without good reason, as it seems to us. When certain tissue becomes specialised for the conveyance of messages from the periphery to the centre, or vice vcrsti, it goes by the name of nom among ani- mals, and the same thing should have the same name in a plant, in spite of the shock it may give to some human nerves to find such a bond of union established. In the sundew, as indeed in the tissue of the Dionoea beyond the filaments, the nerve-force seems to be conveyed in every direction by the ordinary tissue of the plant. Mr. Darwin's experiments show this conclusively. It is not what we should at first sight have expected, especially in the Dionoea, but deeper consideration shows that it is just what ought to be, and that it is another of our author's great dis- coveries in support of evolution. In the individual, nerve-tissue is developed from ordinary protoplasm, and in the lower orders of animal life, as in crustaceans, ordinary non-specialised pro- toplasm must, in greater part, serve for nerve-functions. Thus, when we get to a still earlier phase of the existence of nerve-function, where it has to serve the demand for movements of mere flexion and extension of the most simple and uniform character, specialisation of tissue for the conveyance of currents is not needful, any more than insulation is not needed for a wire when passing through a bad conductor like air. In fact, the analogy is complete. Nerves are only insulated protoplasm, developed when many definite currents are needed and must not be confused by contact. In the fly-trap only one current is used for one purpose, so it may safely go all over the leaf.
We have no space to say much more that we should wish said. Mr. Francis Darwin promises to be a worthy son of a noble father. His contributions to the work are valuable, and especially so are hie splendid representations of various facts by woodcut. The book is got up uniform with most of Mr. Darwin's other works issued by Murray, and it is in Murray's best style. The proofs have been most carefully corrected, as we have noticed only two typographical errors.
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