« PreviousContinue »
instances of the upper surface being hairy while the lower is smooth.
The ramifications of the petiole among the cellular tissue of the leaf are called veins, and the manner of their distribution is termed venation. This influences in a great degree the figure and general appearance of the foliage. The vein which forms a continuation of the petiole and the axis of the leaf is called the midrib or costa : from this all the rest diverge, either from its sides or base. If other veins similar to the midrib pass from the base to the apex of a leaf, such veins have been named nerves ; and a leaf with such an arrangement of its veins has been called a nerved leaf. In speaking of these parts, a leaf is said to be three, or five, or otherwise nerved, if the so-called nerves all proceed from the very base of the lamina, but it is called triple, quintuple, &c. nerved, if the nerves all proceed from above the base of the lamina. If the veins diverge from the midrib towards the margin, ramifying as they proceed, such a leaf has been called a venous or reticulated leaf. This is the sense in which these terms were used by Linnæus; but Link and some others depart from so strict an application of them, calling all the veins of a plant nerves, whatever may be their origin or direction.
The veins are, however, improperly called nerves, either in all cases, as by Link, or in certain cases only, when they have a particular size or direction, as by Linnæus and his followers. Nothing is more destructive of accurate ideas in natural history than giving names well understood in one kingdom of nature to organs in another kingdom of a different kind, unless it is the, perhaps, more reprehensible practice of giving two names conveying different ideas to the same organ in the same kingdom of nature. Thus, when the veins of a plant are termed nerves, it is naturally understood that they exercise functions of a similar nature to those of the nerves of animals : if otherwise, why are they so called ? But they exercise no such functions, being mere channels for the transmission of fluid. Again, if one portion of the skeletori of a leaf is called a vein, and another portion a nerve, this apparently precise mode of speaking leads yet more strongly
to the belief that the structure and function of those two parts are as widely different as the structure and function of a vein and a nerve in the animal economy; else why should such caution be taken to distinguish them? But, in fact, there is no difference whatever, except in size, between the veins and nerves of a leaf.
For the sake of obtaining great precision in describing such a very important and various-formed organ as the leaf, many terms have been invented, especially by Link and De Candolle, which, although not used in daily parlance, are important where brevity and precision are required. Without exactly adopting the nomenclature of either of these distinguished writers, it appears that upon it a system of names may be founded, to which the systematist can have little to object.
It has been usual to call that bundle of vessels only which passes directly from the base to the apex of a leaf the rib, or costa, or midrib. This term should be extended to all main veins proceeding directly from the base to the apex, or to the points of the lobes. There is no difference in size in these ribs; and in lobed leaves, which may be understood as simple leaves approaching composition, each rib has its own particular set of veins.
The midrib (fig. 56. 7) sends forth alternately, right and left along its whole length, ramifications of less dimensions than itself, but more nearly approaching it than any other veins: these may be called primary veins (fig. 56. 3). They diverge from the midrib at various angles, and pass to the margin of the leaf, curving towards the apex in their course, and finally, at some distance within the margin, forming an anastomosis with the back of the primary vein, which lies next them. That part of the primary vein which is between the anastomosis thus described, having a curved direction, may be called the curved vein. Between this latter and the margin, other veins, proceeding from the curved veins, with the same curved direction, and of the same magnitude, occasionally intervene: they may be distinguished by the name of external veins (fig. 56. 1). The margin itself and these last are connected by a fine net-work of minute veins, which I would distinguish by the name of marginal veinlets. From the midrib are generally produced, at right angles with it, and alternate with the primary veins, smaller veins; which may not improperly be named costal veins (fig. 56. 5). The primary veins are themselves connected by fine veins, which anastomose in the area between them; these veins, when they immediately leave the primary veins, I would call proper veinlets (venula propriæ) (fig. 56. 4); and where they anastomose, common veinlets (ven. communes). The area of parenchyma, lying between two or more veins or veinlets, I name with the old botanists intervenium.
These distinctions may to some appear over-refined; but I am convinced that no one can very precisely describe a leaf without the use either of them, or of equivalent terms yet to be invented. With respect to their venation only, leaves may be conveniently arranged under the following heads :
1. Veinless (avenium), when no veins at all are formed, except a slight approach to a midrib, as in Mosses, Fuci, &c. Leaves of this description exist only in the lowest tribes of foliaceous plants, and must not be confounded with the fleshy or thickened leaves common among the higher orders of vegetation, in which the veins are by no means absent, but only concealed within the substance of the parenchyma. (See No. 10.) Of this De Candolle has two forms, — first, his folia nullinervia, in which there is not even a trace of a midrib, as in Ulva ; and second, his folia falsinervia, in which a trace of a midrib is perceptible. These terms appear to me unnecessary; but, if they be employed, the termination nervia must be changed to venia.
2. Equal-veined (æqualivenium), when the midrib is perfectly formed, and the veins are all of equal size, as in Ferns.
This kind of leaf has not been before distinguished : it may be considered intermediate between those without veins and those in which primary veins are first apparent. The veins are equal in power to the proper veinlets of leaves of a higher class.
3. Straight-veined (rectivenium). In this the veins are entirely primary, generally very much attenuated, and arising from towards the base of the midrib, with which they lie nearly parallel: they are connected by proper veinlets ; but there are no common veinlets. The leaves of Grasses and of Palms and Orchidaceous plants are of this nature. This form has been called by Link paralleli- and convergenti-nervosum, according to the degree of parallelism of the primary veins; and to these two he has added what he calls venuloso-nervosum, when the primary veins are connected by proper veinlets : but as this is always so, although it is not in all cases equally apparent, the term is superfluous. Ach. Richard calls this form laterinervium, and De Candolle rectinervium ; from which I do not find it advisable to distinguish his ruptinervium, which indicates the straight-veined leaf, when the veins are thickened and indurated, as in the Palm tribe.
4. Curve-veined (curvivenium). This is a particular modification of the last form, in which the primary veins are also parallel, simple, and connected by unbranched proper veinlets; do not pass from near the base to the apex of the leaf, but diverge from the midrib along its whole length, and lose themselves in the margin. This is the folium hinoideum and venuloso-hinoideum of Link, the f. penninervium of A. Richard, and the f. curvinervium of De Candolle. It is common in Zingiberaceæ. It is supposed by the last named Botanist that both this and the last ought to be regarded as peculiar modifications of petiole (a kind of phyllodia), rather than as true leaves analogous to those next to be described.
5. Netted (reticulatum). Here the whole of the veins which constitute a completely developed leaf are present, arranged
as I have above described them, there being no peculiar combination of any class of veins. This is the common form of the leaves of Dicotyledons, as of the Lilac, the Rose, &c. It is the folium venosum of Linnæus, the f. indirectè venosum of Link, the f. mixtinervium of A. Richard, and the f. retinervium of De Candolle. If the external veins and marginal veinlets are conspicuous, Link calls this form combinatè venosum ; but if they are indistinct, he calls it evanescentè venosum.
6. Ribbed (costatum). In this three or more midribs proceed from the base to the apex of the leaf, and are connected by branching primary veins of the form and magnitude of proper veinlets, as in Melastoma. This must not be confounded with the straight-veined leaf, from which it may in all cases of doubt be distinguished by the ramified veins that connect the ribs. This is a very material difference, which has never been properly explained. Linnæus and his followers confound the two forms; but modern writers separate them : although it must be confessed that it is difficult to discover their distinctions from the characters hitherto assigned to them. Link calls these leaves f. nervata, A. Richard f. basinervia, and De Candolle f. triplinervia and f. quintuplinervia. If a ribbed leaf has three ribs springing from the base, it is said to be three-ribbed (tri-costatum, trinerve of authors); if five, five-ribbed, and so on. But if the ribs do not proceed exactly from the base, but from a little above it, the leaf is then said to be triple-ribbed (triplicostatum), as in the Helianthus.
7. Falsely ribbed (pseudocostatum), is when the curved and external veins, both or either, in a reticulated leaf, become confluent into a line parallel with the margin, as in all Myrtaceæ. This has not been before distinguished.
8. Radiating (radiatum), when several ribs radiate from the base of a reticulated leaf to its circumference, as in lobed leaves. This and the following form the f. directè venosum of Link: it is the f. digitinervium of A. Richard. Hither I refer, without distinguishing them, the f. pedalinervia, palminervia, and peltinervia of De Candolle; the differences of which do not arise out of any peculiarity in the venation, but from the particular form of the leaves themselves.