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COMBINATION OF SPECIES INTO GENERA.

Thus of the Willow, 71 species have been stated by one authority to exist in Britain, whilst another reduces them to 29. The genus Rubus or common Bramble has been thought to contain 21 British species, which are probably reducible to 6 or 8. These details are here introduced, for the purpose of putting the young Botanist on his guard, against the tendency to multiply species, which is now prevalent among superficial writers, and which is still further encouraged by Gardeners, who give new specific names to such varieties as those just alluded to, and even to hybrids between these (§ 454.).

477. When the Botanist has satisfied himself regarding the species which he has collected, his next step is to combine those amongst which he finds the greatest resemblance, into genera. Now in this process he must not be influenced by similarity in their general external aspect; for this will often conceal great differences in their most important organs. There are certain parts which furnish essential characters, without similarity in which it would be wrong to associate species, however alike in other respects, in the same genus; and, on the other hand, there are parts so susceptible of variation, that the differences between them must be very striking indeed, to warrant the plants being arranged under different genera, when they agree in what have been termed the essential characters. Thus, for instance, the general outline of the leaf has been stated to be often subject to great variety, in accordance with the degree in which the space between the veins is filled up with fleshy parenchyma (§ 234,5); and in most cases, a difference in the outline of the leaves of two plants, the distribution of the veins remaining the same, would not alone serve to cause two plants exhibiting it to rank even as distinct species. But any considerable alteration of the veining would be held sufficient for such a separation; though the two plants, if agreeing in the structure of their organs of fructification, would still be placed in the same genus. On the other hand, a marked and constant difference in the organs of fructification would be rightly held sufficient to place the two species in different genera, even though the form and veining of the leaves might be precisely the same.

FORMATION OF ORDERS AND CLASSES.

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On the relative value of the characters furnished by the different organs more will hereafter be stated.

478. Even when thus grouped together into genera, however, the number of objects, which the Botanist has to study, remains by far too great for convenience; and he next forms his genera into orders, and combines these orders into classes, according to their respective correspondence and difference in certain characters of a still more general nature. Now in this process he may follow two very different plans; and upon these are founded the two systems of classification which are now in vogue. The one of these is termed the Linnæan System, after its founder; or the Artificial System, from its character: the other is termed the Natural System. In the Linnæan System, a small number of characters,-chiefly the number of stamens and pistils-is taken as the standard; and the whole Vegetable Kingdom is distributed under classes and orders, according to the correspondences and differences among the several genera in these respects,-no regard whatever being had to any other characters. In the Natural System, all the characters of the genera are studied; and those are united into orders, which present the greatest correspondence in the characters that are regarded as of the most importance: on the same principle, the orders are united into classes. If the former plan be followed, genera most widely differing in their structure and physiological characters are often brought together, and others which are nearly allied are frequently separated to a great extent; so that in fact, it is very common to find, that nothing can be stated of all the plants included in a Linnæan order, except that they have a similar number of stamens and pistils. On the other hand, in the Natural System, the number of characters, in which there is a general agreement among all the plants of a particular order, is so great that, to say that the plant belongs to a certain order, is at once to give the greater part of its description. This is the case also in the highest or most general groups. For instance, to say that a particular species is an Exogen, is at once to make known the structure of its stem and the mode of its increase,-to express the important fact that it has two cotyledons or seed-leaves,-to render it most

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CONNEXION OF STRUCTURE AND PROPERTIES.

probable hat the arrangement of the veins in its leaves is reticulated rather than parallel,-and to intimate that the parts of its flowers are likely to be arranged in fives or fours, rather than in threes.

479. There is a point of agreement among the plants brought together in Natural orders, which is of the greatest practical importance. This is, that those which agree in structure almost invariably correspond in properties also. For instance, the whole of the Papaveracea or Poppy tribe possess narcotic properties; all the Ranunculacea or Crowfoot tribe are acrid; whilst all the Malvacea or Mallow tribe are destitute of unwholesome properties. Thus, when a plant is recognized as a member of a particular Natural Order, an almost certain account may be given of its properties,—whether it is likely to be injurious or wholesome, to furnish valuable medicines, or important articles of food. It must be remembered, however, that the peculiar properties of the plant do not pervade every portion of it; and that it may hence be possible to obtain wholesome nutriment, even from members of orders most distinguished for their deleterious properties. Thus the Potatoe belongs to the order Solanea, which contains the Deadly Night-shade, Henbane, and other poisonous plants; but the edible part of it, which is a deposition of starch for an express purpose (§ 337), is free, or nearly so, from the narcotic properties which exist in the stems and leaves. Indeed, as a general rule, such depositions of starch may supply wholesome food in any order, more especially if care be taken to free them from any juices they may contain; thus the Cassava, which furnishes one of the most important articles of food to the inhabitants of many tropical countries, is obtained from'a plant of the order Euphorbiacea or Spurge tribe, which is distinguished for its very acrid qualities: and these are shared by the juice expressed from the meal after it has been ground.

480. The Linnæan system, however, is not without its advantages for particular purposes. To a person commencing by himself the study of Systematic Botany, desirous of making himself acquainted with the names and characters of the plants he may

USE OF THE LINNEAN SYSTEM.

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meet with in his walks, and not ambitious of extending his studies to the higher parts of the science, the Linnæan system, when applied with the aid of books, possesses facilities which are (at present at least) greatly superior to those afforded by the other, and which are well calculated to encourage a learner. To count the number of stamens and pistils is generally a very easy process; this at once establishes the class and order; and nothing then remains, but to determine the genus and species, which (among the comparatively small number found in Britain) a little practice in the examination of characters will enable any intelligent person to do, with the aid of books in which these are laid down. The habit thus gained of discriminating characters, and of applying terms, is a most valuable preparation for the study of the Natural System when opportunity presents itself. It must be constantly borne in mind, however, that the utmost use that can be made of the Linnæan system, consists in the assistance it affords in the discovery of the name of an unknown plant; and, until this has been made out, the previous determination of its class and order gives no indication of its general structure and properties (not even making it apparent whether it is an Endogen or an Exogen, a Dicotyledon or a Monocotyledon), since under the same head are grouped genera of the most opposite character. It may be said that it serves as a sort of Alphabetical index to a book, enabling the reader to turn to any part of it he wishes, by looking out the subject in the order of its first letters, but giving no idea whatever of the general scope of the book, nor of the mode in which its subjects are arranged.

481. The Linnæan System is liable to many imperfections and difficulties in its application, even in the limited circle of British Plants; for example, the number of pistils is liable to be altered in any species by the more or less complete adhesion of the carpels; and that of the stamens may also vary in the different species of the same genus, and even among the different individuals of the same species, or even (in some instances) among the different flowers of the same stem. The adoption of characters thus liable vary cannot, therefore, but sometimes lead to confusion. For

to

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IMPERFECTIONS OF THE LINNEAN SYSTEM.

instance, of the genus Polygonum, of which the several British species are known by the names of Bistort, Buck-wheat, Persicaria, &c., one has always, and two others have occasionally, eight stamens; whilst in the rest the number varies from five to ten. As eight seems to be the most regular number, the plant is placed in the class Octandria: and although its styles are sometimes only two in number, it is placed in the order Trigynia, because they are more commonly triple. Now if a student meet with a specimen which has five, six, or ten stamens, he will vainly search for its character among the genera of the Linnæan class to which it would seem to belong; and unless he happen to consult a book which makes special mention of the genus in these several classes, he will be altogether at fault. Suppose that some more knowing Botanist tells him that his plant is a Polygonum, he will again turn to his book, wondering how he could have overlooked it ; but he will find the genus in the class Octandria, in spite of the different number of stamens in the specimen before him: and he will then learn that it is placed in the genus Polygonum on account of its strong general resemblance to other Polygonums, in spite of its differing from them in characters which are ordinarily considered as sufficient to establish classes and orders. Again, the greater part of the species of the genus Rhamnus (Buckthorn) possess both stamens and pistils in the same flower; but the species most commonly known in this country on account of its purgative properties is Dioecious, the stameniferous flowers being on one plant, and the pistiliferous on another. (§ 435). The students who met with it, therefore, would seek for it in the class Diœcia, where he would be disappointed as before; since, as in most species the flowers are complete, it is placed in the class and order to which the number of its stamens and pistils would refer it.

482. Such exceptional cases occur much more frequently than is commonly supposed. It has been proved that, in fourteen divisions of the Linnæan system, including 173 British genera, there are no less than 43 exceptions,-one-quarter of the whole; and that out of 274 genera of North America, belonging to eighteen Linnæan sections, there are 78 exceptions,-rather more than a quarter.