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colour, and connected with the cavernous parenchyma of the pitcher. Of these more will be said under the head of GLANDS.
3. Of Hairs.
These (fig. 15.) are minute, transparent, filiform, acute processes, composed of cellular tissue more or less elon
gated, and arranged in a single row. They are found occasionally upon every part of a plant, even in the cavities of the petiole and stem, as in Nymphæa and other aquatic plants. In the Cotton Plant (Gossypium herbaceum, &c.) they form the substance which envelopes the seeds, and is wrought into linen; in the Cowhage (Mucuna urens and pruriens), it is they which produce the itching. In Ferns they are long, entangled, strangulated filaments. They vary extremely in length, density, rigidity, and other particulars; on which account they have received the following names:
Down or Pubescence (pubes, adj. pubescens), when they form a short soft stratum, which only partially covers the cuticle, as in Geranium molle.
Hairiness (hirsuties, adj. hirsutus), when they are rather longer and more rigid, as in Galeopsis Tetrahit.
Pilosity (adj. pilosus), when they are long, soft, and erect, as in Daucus Carota.
Villosity (adj. villosus), when they are very long, very soft, erect, and straight, as in Epilobium hirsutum. Crini (adj. crinitus) are this variety in excess.
Velvet (velumen, adj. velutinus), when they are short, very dense and soft, but rather rigid, and forming a surface like velvet, as in many Lasiandras.
Tomentum (adj. tomentosus), when they are entangled, and close pressed to the stem, as in Geranium rotundifolium.
Cilia (adj. ciliatus), when long, and forming a fringe to the margin, like an eyelash, as in Sempervivum tectorum.
Bristles (setæ, adj. setosus), when short and stiff, as on the stems of Echium.
Stings (stimuli, adj. stimulans; pili subulati of De Candolle), when stiff and pungent, giving out an acrid juice if touched, as in the Nettle.
Glandular hairs (pili capitati), when they are tipped with a glandular exudation, as in Primula sinensis. These must not be confounded with stalked glands.
Hooks (hami, unci, rostella), when curved back at the point, as in the nuts of Myosotis Lappula.
Barbs (glochis, adj. glochidatus), if forked at the apex, both divisions of the fork being hooked, as in the nuts of the same plant.
Hairs also give the following names to the surface of any thing:
Silky (sericeus), when they are long, very fine, and pressed closely to the surface, so as to present a sublucid silky appearance: ex. Protea argentea.
Arachnoid, when very long, and loosely entangled, so as to resemble cobweb: ex. Calceolaria arachnoidea.
Manicate, when interwoven into a mass that can be easily separated from the surface: ex. Cacalia canescens, Bupleurum giganteum.
Bearded (barbatus), when the hairs are long, and placed in tufts: ex. the lip of Chelone barbata.
Rough (asper), when the surface is clothed with hairs, the lower joint of which resembles a little bulb, and the upper a short rigid bristle: ex. Borago officinalis.
Stellate, or starry, when the hairs grow in tufts from the surface, and diverge a little from their centre, as in the Mallow tribe.
Hairs are either formed of a single cell of cellular tissue (Plate I. fig. 8. b, and Plate II. fig. 18.) or of several placed end to end in a single series, (Plate I. fig. A, B,) whence, if viewed externally, they have the appearance of being divided internally by transverse partitions. They are sometimes branched into two or three forks at the extremity, as in Alyssum, some species of Apargia, &c. Occasionally they
emit little branches along their whole length: when such branches are very short, the hairs are said to be toothed or toothletted, as in the fruit of Torilis Anthriscus; when they are something longer, the hairs are called branched, as in the petioles of the gooseberry; if longer and finer still, the hair is pinnate, as in Hieracium Pilosella; if the branches are themselves pinnate, as in Hieracium undulatum, the hairs are then said to be plumose. It sometimes happens that little branchlets are produced on one side only of a hair, as on the leaves of Siegesbeckia orientalis, in which case the hair is called one-sided (secundatus); very rarely they appear upon the articulations of the hair, which in that case is called. ganglioneous. (Plate I. fig. 9. Verbascum Lychnitis): the poils en goupillon of De Candolle are referable to this form. Besides these, there are many other modifications: hairs are conical, cylindrical, or moniliform, thickened slightly at the articulations (torulose), as in Lamium album, or much enlarged at the same point (nodulose), as in the calyx of Achyranthes lappacea. In Polystachya luteola the hairs of the labellum are moniliform, or necklace-shaped, with the articulations all spheroidal, equal sized, and disarticulating at the slightest touch when the flower is expanded, so that the part on which they grow seems as if it were covered with fine powder.
Hairs are sometimes said to be fixed by their middle (Plate I. fig. 10. c); a remarkable structure, common to many different genera: as Capsella, Malpighia, Indigofera, &c. This expression, however, like many others commonly used in botany, conveys a false idea of the real structure of such hairs. They are in reality formed by an elevation of one bladder of the epidermis above the level of the rest, and by developement of a simple hair from its two opposite sides. Such would be more correctly named divaricating hairs. When the central bladder has an unusual size, as in Malpighia, these hairs are called poils en navette (pili Malpighiacei) by De Candolle, and when the central bladder is not very apparent, poils en fausse navette (pili pseudo-Malpighiacei, biacuminati), as in Indigofera, Astragalus asper, &c. In many plants the hairs grow in clusters, as in Malvaceæ, and
are occasionally united at their base: such are called stellate, and are frequently peculiar to certain natural orders. (Plate I. fig. 10. a.)
All these varieties belong to one or other of two principal kinds of hairs; viz. the Lymphatic and the Secreting. Of these, lymphatic hairs consist of tissue either tapering gradually from the base to the apex, or at least not much enlarged at either end; and secreting, of cellules visibly distended either at the apex or base into receptacles of fluid. Malpighiaceous and glandular hairs, stings, and those which cause asperity on the surface of any thing, belong to the latter; almost all the other varieties to the former.
When hairs arise from one surface only of any of the appendages of the axis, it is almost always from the under surface; but the seed-leaves of the nettle, and the common leaves of Passerina hirsuta, are mentioned by De Candolle as exceptions to this rule: certain states of Rosa canina might also be mentioned as exhibiting a similar instance. When a portion only of the surface of any thing is covered by hairs, that portion is uniformly the ribs or veins. According to De Candolle, hairs are not found either upon true roots, except at the moment of germination, nor upon any portion of the stem that is formed under ground, nor upon any parts that
grow under water.
In a very large number of hairs, perhaps in all, there may be seen, at some period of their existence in any cell, a cytoblast, and a circulating system, formed of numerous fine streams, which all appear to proceed from and return to the cytoblast itself. (See Plate II. fig. 13. 14. 18.) In the moniliform disarticulating hairs of Polystachya, already described, each joint of the hair has this structure in a very remarkable
If hairs are examined with low magnifying powers, their sides appear to be simple, and they are accordingly regarded as mere expansions or attenuations of the vesicles of the epidermis: but if they are studied with more attention and more powerful microscope, it becomes evident that their sides are double; for currents may often be seen streaming along their sides, and evidently interposed between the external
smooth surface and an uneven interior membrane. This is easily observed in the jointed hairs of Tradescantia virginica, where the nature of the current is distinctly shown by minute molecules, that are carried along by the stream. If the hair of Tradescantia is suffered to die on the field of the microscope, and dry up, it then becomes evident that it is composed of two sacs, the one firm and external, the other extremely thin, and after death contracting so much as to leave a considerable space between its sides and the external sac. (See Plate II. fig. 14. b.) It appears to me that this is the general structure of all hairs in which a circulation of sap, and the cytoblast are both visible; and it is probable that the external sac is the cuticular membrane, hard, firm, and scarcely capable of shrivelling; while the internal sac is a cell of the parenchyma, thin-sided, and not acquiring any firmness with age, but shrivelling up as soon as the fluid which distends it when alive is withdrawn.
4. Of Scurf.
SCURF consists of thin flat membranous disks, with a ragged margin, formed of cellular tissue, springing from the epidermis. It may be considered as a modification of hairs; for it differs from those bodies only in being more compound. It is of two kinds, Scurf, properly so called, and Ramenta.
Scurf, properly so called, are the small, roundish, flattened, particles which give a leprous appearance to the surface of certain plants, as the Elæagnus and the Pine Apple. (Plate I. fig. 10. b.) They consist of a thin transparent membrane, attached by its middle, and, owing to the imperfect union, towards its circumference, of the cellular tissue of which it is composed, having a lacerated irregular margin. A scale of this nature is, in Latin, called lepis, and a surface covered by such scales lepidotus -not squamosus, which is only applied to
a surface covered with the rudiments of leaves. Scurfs are the poils en écusson (pili scutati) of De Candolle.
Ramenta (Vaginella) are thin, brown, foliaceous scales, appearing sometimes in great abundance upon young shoots.