« AnteriorContinuar »
plant-structures are in adaptation to their needs. For when one studies the structures and functions of roots, stems, leaves, flowers and fruits, it becomes perfectly clear that these structures and their functions are in harmony with their surroundings; that is to say, e.g., the root is adapted for absorbing water, mineral, and other matters from the soil; that the stem is adapted to support the plant or tree and convey the fluids upwards; that the leaves are adapted to imbibe carbonic acid from the air and to decompose it and then convert it into starch, as well as to transpire or exhale large quantities of superfluous moisture; also to respire, as we do ; for it is as necessary for plants to breathe as much as animals, and if they be prevented doing so they will be asphyxiated too. Then, again, all the minute details of the structure of flowers are so many adaptations to the requirements of setting seed, by which the plant can be reproduced.
Hence we can recognise two “ ends” in plant-life, viz., a healthy vigorous growth of the individual, and the production of plenty of good seed on the event of its death. We soon see, therefore, how every phase of its existence is in harmony with, or “correlated” to its environment, and under that term is included soil and its ingredients, air, moisture, temperature, light, &c.; and in the case of flowers, the visits of particular insects, if they be required, as is often the case, if not, then it is the wind, or else the flowers are so formed as to be independent of both wind and insects, and are specially constructed to be able to fertilise themselves. Such is the environment taken in its widest sense, and we repeat the question : How has any plant become adapted to its environment, so that all its various organs can perform their several functions in perfect harmony with the various features of this environment ?
There are two ways of answering this question. First, to examine plants in nature which can, and often do, live under very different conditions or surroundings; as, e.g., under water or on land and in air, and watch them to see how they change. These might be called natural experiments. We may also cultivate plants in very different and artificially prepared soils, &c., from those in which they naturally grow in the wild state. Such will be artificial experiments.
Secondly, there is a line of argument of great value, which will be emphasized further on by illustrations. It is, that when innumerable cases occur of certain phenomena always re-appearing under similar circumstances or conditions, we are justified in drawing the conclusion that there is a distinct cause and effect, even though we may not be able to verify our inference or deduction by actual experiment. In many cases we can, it is true, supply the conditions, and then we discover that we can produce at will the same effects as those we see appearing in nature under similar circumstances. This of course greatly strengthens the argument, but it is not absolutely necessary for the establishment of the truth.
THE ENVIRONMENT OF PLANTS.
As a good example of one of Nature's experiments, let us take the water Crowfoot (Ranunculus aquatilis). This plant grows in ponds and rivers, &c. In still water it produces very finely divided leaves with almost thread-like divisions, all of which are always entirely submerged; but when the stem grows to near the surface of the water, it develops ordinary leaves, heartshaped or rounded with a scolloped edge. These float on the surface.
When we examine the tissues of these leaves microscopically, there are the following differences: in the floating leaves there is a superficial cellular skin or epidermis, which is composed of a layer of colourless cells like flat boxes, with a wavy contour all fitting tightly together like the pieces of a mappuzzle. At intervals there are an abundance of “ breathingholes,” or stomata. They are composed of two oblong cells, rounded at the ends and curved towards each other, leaving a space between them for the circulation of air within and without the leaf. The stomata are on the upper epidermis exposed to the air. There are none, or at least very rarely one or two, here and there on the lower epidermis in contact with the water. The intermediate tissue contains the green granules called chlorophyll grains.
In the submerged leaves there is no true epidermis; the outermost layer of cells, which represents it, is full of green chlorophyll, and there are no stomata at all.
There are, indeed, several other differences which need not be specified. The conclusion is that the one kind of leaf is adapted to water, the other to float on the surface and to have at least one epidermis adapted to air.
Now suppose the pond to dry up gradually during a hot summer, so that the water all goes, leaving nothing but mud, which is in the course of drying up too. What happens ? Our Ranunculus becomes a terrestrial plant. It produces an abundance of " dissected" leaves, very similar in appearance to the submerged ones, but not at all identical in structure. The little thread-like divisions now grow stouter and can stand erect in air. They are a little flattened instead of being circular in a cross section. They develop plenty of stomata, and possess a true epidermis, which, as is usual with aërial leaves, has no chlorophyll except in the cells of the stomata.
Here, then, we recognise two facts. One, that heredity compels the plant to produce leaves like the submerged ones, but the new environment compels it to construct the leaves for an aërial existence. Sometimes only this sort of leaf is produced, and the Ranunculus then carpets the ground with a kind of soft green pile. . Sometimes it produces a certain number of dissected leaves, and then suddenly changes and bears the form of leaf which floats. Its habit was too strong to be lost. As these leaves are already partially adapted to air, they do not alter, their structure being just the same as when floating.
Here, again, heredity comes into play, for ordinary leaves of plants and trees have stomata chiefly or only on the lower and not the upper side. Hence the fact that the water Crowfoot has them on the upper and not the lower is clearly an adaptation to their floating on water, because the latter element prevents their formation on the lower side.
If the aërial plant be now transformed back again to water and submerged, all the "aërial " leaves perish, but it throws out fresh “aquatic ” leaves instead. Hence one cannot avoid the conclusion that in some way or other an aquatic medium is the direct cause of one set of structures, and the aërial that of another set. Of course it is not only the difference between water and air, but all the concomitant circumstances associated with these two media respectively, which make up the environment and take effect upon plants. Thus the greater amount of light in air than in water acts most powerfully in regulating the structure of the tissues; the difference in the amount of carbonic acid, &c., all, no doubt, assert their influence; so that the different effects produced in plants are the result of the combination of several phenomena acting together.
In the continuation of this subject I propose to deal with a theoretical origin of Floral Structures.
(To be continued.) [We have much pleasure in inserting the foregoing able article from Professor Henslow, who is an eminent scientific authority, and a valued supporter from its foundation of the Selborne Society, of which he is a Vice-President. It must not, however, be supposed that NATURE Notes is in any way committed to Evolutionist principles. An article on the other side, written with equal knowledge and a similar absence of the polemical spirit, would be just as willingly inserted in our columns.]
THE PRESERVATION AND ENJOYMENT OF
account of the functions of the three Societies having
ment of open spaces—the Commons Preservation Society, the Kyrle Society, and the Metropolitan Public Gardens Association. The movement took its rise in the necessity of securing for public use the beautiful commons which lie on the outskirts of London, but it has taken effect in many directions since the formation of the Commons Preservation Society in 1865. I will briefly indicate the several questions which have arisen from time to time.
And first as to common land. Commons are beset by three dangers. They may be arbitrarily inclosed by the lord of the manor
PRESERVATION OF OPEN SPACES.
under some claim of right. In this case the fight must be fought in the Law Courts, and it must be fought by local champions and with local weapons. The Open Space Societies cannot appear as litigants, but they all, and particularly the Commons Preservation Society, give advice and aid to the commoners in such case. Secondly, commons may be inclosed on the recommendation of the Board of Agriculture (Successors to the Inclosure Commisioners). Here the sanction of Parliament must be obtained, and the Commons Preservation Society carefully watches every proposal of the kind, with the result, as I have already stated, that inclosure is practically at an end. Thirdly, commons may be appropriated by railway companies, promoters of water-works and other industrial undertakings. Some years ago a railway engineer eagerly sought for common land in the country he had to traverse, and took his line through every tract of open waste he could find, for it was cheaper to buy such land than inclosed land. Many commons round London have been sliced and marred by railways—Wandsworth and Tooting Commons and Banstead Downs are notable sufferers. Wimbledon, Clapham and Hampstead have been saved from a like fate only by strenuous opposition. In those days the public generally had no notice of the intention to carry a line through common land; it was only by accident that the proposal was discovered before it was too late. Now, in consequence of an alteration in the standing orders of Parliament—made at the instance of Mr. Shaw Lefevre and the Commons Preservation Society-promoters of railways must give notice in the London Gazette of every proposal to take common land. Since this change every attempt sericusly to encroach has been defeated, and railway engineers are recognising that it is a dangerous and costly step to threaten land which is open to the use and enjoyment of the public.
The principle has indeed been extended far beyond common land. Disused burial grounds and square gardens have been brought within its scope. Only this year the London and North Western Railway Company were defeated in an attempt to take part of Euston Square, and in 1889 the Midland Company were allowed to acquire a small piece of a burial ground at St. Pancras only on condition that they gave the County Council an equivalent in land, or in money to be spent in acquiring other land. Whole districts also have been protected from disfigurement by railway companies. Several attempts have been made to carry railways through the Lake district, but at the instance of the Open Space Societies they have been defeated, and similar protection has been accorded to the New Forest.
This is only one instance of the broad and at the same time reasonable spirit in which the Open Space Societies have interpreted their duties. The Commons Preservation Society, moved to the work and aided by the Society for the Protection of Ancient Buildings, a few years since defeated an endeavour to demolish
the interesting and venerable buildings of the London Charterhouse, and to build over the gardens and quadrangles with which Thackeray familiarised all England. Staple Inn, again, and Barnard's Inn—two of the Inns of Chancery which once played an important part in the education of lawyers-have been, so far, saved from the hand of the destroyer mainly through the exertions of the same Societies. And at the present moment the Open Space and Ancient Building Societies are seeking to prevent the sacrifice of Emanuel Hospital, Westminster-Lady Dacre's kindly foundation-to that spirit of false utilitarianism which sometimes intrudes into the management of ancient charities.
The attempt to manage Crown lands with a view to pecuniary profit alone has also been stoutly opposed by the Open Space Societies. Mr. Fawcett was the first boldly to lay down the principle that in such a case as the New Forest the interests of the nation were better served by the preservation of a national pleasure-ground than by the slight increase of revenue which might possibly arise from enclosure and tillage. At the present moment it is sought to apply this principle to Sudbrook Park, Richmond, an appanage to Richmond Park, to the full enjoyment of which its preservation intact largely conduces.
Sometimes, however, there is nothing for it but to buy the land which is required for the public enjoyment. In such cases the first move is generally made by one or other of the Open Space Societies, and a special committee is subsequently formed. The splendid tract of land lying between Hampstead Heath, and Highgate, Clissold Park, Stoke Newington, and the Lawn at South Lambeth—for some years the home of the late Mr. Fawcett—have thus been rescued from the builder, while a similar movement respecting the Hilly Fields at Lewisham is not yet assured of success. In these cases the rates, metropolitan and local, the funds of the City parochial charities, and private purses have all alike been laid under contribution, and skill and judgment were required to adjust means to ends, and conduct the purchase to a successful issue.
ROBERT HUNTER. (To be continued).
SYMPATHY OF BIRDS WITH THEIR KIND.
HERE is no doubt as to the cruelty shown by wild
birds and beasts to suffering and feeble members of their own kind. It is a factor, not to be ignored, in
the all-important law of the survival of the fittest. We read an instance of it in Miss Durham's interesting little account of the deserted jackdaw in last month's magazine. But, as Schrader well shows in his Prehistoric Antiquities of the Aryan Peoples, we must not forget that man himself in his primitive barbarous state did not scruple to kill (and often eat) useless infants, and feeble individuals of his own tribe.