Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/101

Different organs of plants are transformed into hairs, as may be seen in the flowering stalks of the Wig-tree (ilhus

Cotinus), and in the calyx of Compositor.

Names are given to the surfaces of plants according to the presence or absence of hairs, as well as the nature of the hairs which cover them The following are the more important terms Glabrous, smooth, having no hairs ; hairy or pilose, furnished with hairs ; pubescent, covered with soft, short, downy hairs ; villous, having long, weak, often obliquo hairs : sericeous, covered with long, closely appressed hairs, having a silky lustre ; hispid, covered with long harsh or stiff hairs not appressed , hirsute, having long tolerably distinct hairs, not harsh nor appressed; velvety or velutinous, with a dense covering of short down, like velvet ; tomentose, covered with crisp, rather rigid, entangled hairs like cotton, which form a sort of felt (tomentum) ; woolly, with long curled and matted hairs like wool ; bearded or stupose, when hairs occur in small tufts. The hairs which are most frequently met with in plants are called lymphatic, from their not being connected with any peculiar secretion. Those, on the other hand, which have secreting cells at their base (fig. 43) or apex, are denominated (jlandidar, and are not to be distinguished from glands. On young roots cellular projections occur, which may be called radical hairs. Young leaves and buds are frequently covered with protecting hairs. On the parts of the flower, as in the Iris, coloured hairs occur which have been called coralline.

Fig. 45.

FIG. 44. Radiating hair or scale from Klamfrnus (Oleaster). Fia 45. Peltate hair pp of Malpighia went rising from e, epidermis, $r,glan4 FIG. 40. Irritable leaves of Venus s Fly-trap (Dionua)

Fig. 47.
Gland of Dittany cut vertically.

Fig. 49. Fig. 50. FIG. 48.-- -Superficial glands of the Hop, containing a resinous secretion called Lupulin. FIG. 49. Cluster of ovate-oblong glands from the base of the stipule of the Ipccacuan plant (CephaeHs Ipecacuanha). FIG. 50. One of the petals of Fritillaria imperialis, Crown Imperial, showing ft pit or depression g at the base containing a honey-like secretion. This gland is sometimes called a nectary.

In connection with the epidermal appendages we may

notice glands, although they may occur in any tissue. A gland consists of a single cell or a collection of cells secreting substances different from those contained in the surrounding cells. In the former case the gland is simple, in the latter it is compound. In compound glands it frequently happens that the walls of the inner cells are absorbed, and thus the gland has only a single cavity, as in the glands of the Orange rind (fig. 28) ; these are termed vesicular. The secretion of the glands may be stored in their interior, as in Orange rind, and in the leaves of Laurus Camphora, or it may be exuded as in Lychnis viscaria, and in the nectaries of Fritillaria imperialis (fig. 50). Hairs serve as ducts through which the secretion of glands is dis charged. Such hairs are seen in the Nettle (fig. 43), in Loasa or Chili Nettle, and in Malpighia (fig. 45), and are commonly called stings (stimuli). In the Nettle they are formed of a single conical cell, dilated at its base, and closed at first at the apex by a small globular disk placed obliquely. This disk breaks off on the slightest touch, when the sharp extremity of the hair enters the skin, and pours into the wound the irritating fluid which has been pressed out from the elastic epidermal cells at the base. When a nettle is grasped with violence, the sting is crushed, and hence no injury is done to the skin. The glandular en larged apex of hairs sometimes exudes a viscid secretion, as in the Chinese Primrose and in the Sundew. The hairs of the latter plant, by this secretion, detain insects which happen to alight on them. The hairs gradually close on the insects, electrical phenomena taking place during the movement, and then the secretion from the glands becoming acid, an action takes place upon the organic matter analogous to that of the pepsin of the gastric juice, by which it is rendered soluble, and is eventually absorbed by the plant. A similar property is possessed by the secretion from the glands upon the surface of the leaf of Venus s Fly-trap (Dionaea muscipula, fig. 46). The acid in both instances belongs to the formic acid series. Glands may be either internal or external, and they may be situated at the extremity of a hair (when they are stalked), or they may be immersed in the substance of the plant (sessile). In the Dittany (Dictamnus albus, fig. 47) a form of gland is seen intermed iate between the sessile and stalked form. The glands in this plant secrete a green oily matter, so also do the stalked glands in the Hose. In the Ice-plant the glands appear as elevations of the epidermis, containing a transparent fluid like ice, which is said to have an alkaline reaction ; in the Chick-pea similar superficial cells contain an acid fluid. Clear glands are also seen on the under surface of the leaf of Passiflora lunata. Resinous glands are seen in the Hop (fig. 48) and Hemp plants. At the base of the petals of the Crown-imperial (fig. 50), cavities occur containing a honey- like fluid, secreted by what are called nectariferous glands. Cavities containing saccharine matter, surrounded by small thin-walled cells, are met with in the leaves of Acacia longifolia, also in Viburnum Tinus, and Clerodendron fragrans. The cavities communicate with the surface of the leaves by means of canals. Peculiar sessile glands secreting a gummy substance are found at the inner side of the base of the petioles of Cinchona and Ipecacuan plants (fig. 49). On the buds of various trees peculiar glandular hairs termed colleters exist, secreting a gummy mucilaginous) matter, the blastocolla, which covers the bud. These,

however, disappear on the bursting of the bud. They