HAIRS. 



[ 380 ] 



HAIRS. 



one or more of the component cells filled 

 with special oily, resinous, or saccharine 

 secretion. In the latter condition they are 

 termed glandular hairs. The characters of 

 these organs are spoken of under the head 

 of GLANDS and SECRETING ORGANS of 

 PLANTS. The gum-resinous secretion found 

 upon the buds of trees (^Esculus, &c.) is 

 formed by glandular hairs. 



Some of the hairs with watery cell-con- 

 tents present favourable opportunities for 

 observing the ROTATION of the protoplasm; 

 for example, the young hairs of the stamens 

 of Tradescantia or spider-wort; the sting- 

 ing hairs of nettles also show this when 

 young ; and probably it might be observed 

 in all young hairs, where sufficiently trans- 

 parent and uninjured. One precaution 

 greatly facilitates the observation namely, 

 to dip the hairs into alcohol for an instant, 

 find immediately plunge them in water ; 

 after this operation, the structure is readily 

 wetted by water, and no longer obscured by 

 the abundance of air-bubbles that remain 

 entangled with and adherent to the surface 

 of the fresh hairs. These young hairs like- 

 wise exhibit at their apices the various con- 

 ditions of the contents (nucleus, protoplasm, 

 &c.) of cells multiplying by division (PI. 47. 

 figs. 8 & 9). The circulation takes place in 

 the dark streaks represented as forming a 

 network connected with the nuclei (n). 



Stings, such as those of the Nettle (PI. 28. 

 fig. 8), consist of simple cells having a 

 bulbous base enclosed in a cellular case, 

 formed by the growing-up of the epidermis 

 round the base of the hair; the latter tapers 

 away upward to near the apex, where it 

 again expands into a little globular head. 

 The walls are rather thick and spirally stri- 

 ated. The bulbous base is filled with the 

 irritating liquid, which exudes when the 

 knob-like head is broken off, through the 

 tension of the cellular investment of the 

 sac. 



The intimate structure of the hairs of 

 plants presents many points of interest. The 

 cells are of course composed of a cellulose 

 wall, with contents varying according to 

 age and other circumstances. When young, 

 they are always densely filled with proto- 

 plasm (PI. 47. figs. 8 & 9), which becomes 

 gradually excavated by vacuoles, and ex- 

 panded so as to form a mere reticulation or 

 a few streaks upon the wall, mostly con- 

 nected with an evident nucleus. The cavity 

 of the cell is then tilled, in hairs proper, with 

 watery cell-sap, sometimes coloured, as in 



the petals and stamens of many flowers, by 

 the same liquid colouring-matter as the cells 

 beneath the epidermis ; stings are filled with 

 acrid watery juice, glandular hairs with 

 various secretions, which, like the watery 

 juices, appear at first in vacuoles, gradually 

 occupying the place of the protoplasm 

 which follows the expanding cell-walls. 



Hairs, being epidermal structures, possess 

 a more or less evident cuticular layer, which 

 may be detached by the action of acids 

 (fig. 199, p. 295); sulphuric acid often 

 causes this to separate and expand as a kind 

 of vesicle from the surface of the hair, as is 

 shown in PI. 28. fig. 13 (Siphocampylus) ; 

 the cuticle of the full-grown moniliform 

 hairs of Tradescantia may be separated in 

 like manner (see EPIDERMIS). This cuticle 

 also exhibits in many cases the same mark- 

 ings which occur on the surface of the 

 epidermis of certain plants, as Ifelleborus, 

 Cakile, &c. (PI. 28. figs. 9 & 10), consisting 

 of elevated spots, ridges, reticulations, &c. 

 composed entirely of thickenings of the 

 cuticular layer. This is well seen in the 

 hairs of the Boraginaceae, e. g. Anchusa 

 (fig. 17), the Cruciferae, as of Farsetia, 

 Cheiranthus, &c v or Delphinium (fig. 16). 

 The spiral striae on the sting of Urtica urens 

 (fig. 8) appear to be of similar nature. 

 T. West has described the raised markings 

 upon some hairs as bulgings or wrinkles in 

 the cell-wall. 



Finally, it is necessary to mention the 

 remarkable structure of the hairs upon the 

 surface of the seeds and pericarps of certain 

 plants among the Acanthaceae, Polemo- 

 niaceae, Labiatae, Compositae, &c. Those 

 of the ACANTHACEAE have been spoken of 

 partly under that head and under ACAN- 

 THODIUM. They are hairs composed of 

 cylindrical cells, simple (JRuellia, PI. 28. 

 fig. 21), or conjoined into a compound and 

 branched hair (Acanthodium, fig. 24), the 

 cell-walls of which receive when young 

 a spiral (fig. 24) or annular (fig. 21) fibrous 

 deposit, and subsequently become partially 

 disorganized ; so that, if placed in water in 

 the mature state, the primary cell-wall 

 almost dissolves into a kind of jelly, and 

 the spiral-fibrous structure expands with 

 elasticity. The conditions are similar in 

 Collomia (fig. 22), and, according to Schlei- 

 den, in Gilia, Ipomopsis, Polemonium, Can- 

 tua, &c. among the Polemoniacese ; and 

 somewhat the same in many species of Sal- 

 via (fig. 23), Ocymum, Dracocephalum mol- 

 davicum, &c. among the Labiatae. In Cobcea 



