HAIRS. 



[ 314 ] 



HAIRS. 



Some of the hairs with watery cell-con- 

 tents present favourable opportunities for ob- 

 serving the ROTATION of the protoplasm; for 

 example, the young hairs of the stamens of 

 the Tradescantia (or spider-wort of gar- 

 deners) before they have acquired their mo- 

 niliform character and dark contents; the 

 stinging 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, 

 and 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 (PL 38. 

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

 the dark streaks represented as forming a 

 network connected with the nuclei (ri). 



Stings, such as those of the Nettle (PI. 21. 

 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 protoplasm 

 (PL 38. figs. 8 & 9), which becomes gradu- 

 ally excavated by vacuoles, and expanded 

 so as to form a mere reticulation or a few 

 streaks upon the wall, mostly connected with 

 an evident nucleus. The cavity of the cell 

 is then filled, 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, ap- 

 pear 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. 203. p. 237) ; sulphuric acid often causes 

 this to separate and expand as a kind of vesicle 

 from the surface of the hair, as is shown in 

 fig. 13. PL 21 (Siphocampylus) ; the cuticle 

 of the full-grown moniliform hairs of Tra- 

 descantia may be separated in like manner 

 (see EPIDERMIS). This cuticle also exhi- 

 bits in many cases the same markings which 

 occur on the surface of the epidermis of cer- 

 tain plants, as Helleborus, Cakile, &c. (PL 21. 

 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 (PL 21. fig. 17), the Cruciferae, 

 as of Farsetia, Cheiranthus, &c., or Delphi- 

 nium (PL 21. fig. 16). The spiral strige on 

 the sting of Urtica urens (PL 21. fig. 8) ap- 

 pear to be of similar nature. 



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 ACANTHACE^E have been spoken of 

 partly under that head and under ACANTHO- 

 DIUM. They are hairs composed of cylin- 

 drical cells, simple (Ruellia, PL 21. fig. 21), 

 or conjoined into a compound and branched 

 hair (Acanthodium, PL 21. fig. 24), the cell- 

 walls of which receive when young a spiral 

 (fig. 24) or annular (fig. 21) fibrous deposit, 

 and subsequently become partially disor- 

 ganized; 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 ( PL 2 1 . 

 fig. 22), and according to Schleiden, in Gilia, 

 Ipomopsis, Polemonium, Cantua, &c. among 

 the Polemoniaceae, and somewhat the same in 

 many species of Salvia (PL 21. fig. 23), Ocy- 

 mum, Dracocephalum moldavicum,&c. among 

 the Labiatse. In Cobcea scandens, the spiral- 

 fibrous hairs take rather the form of minute 

 scales, and they do not spontaneously ex- 

 pand elastically (PL 21. fig. 20). Among 

 the Compositae, these spiral-fibrous hairs 

 have been observed on the pericarp of Ruc- 

 keria, some species of Trichocline, Euriops, 

 Mesogramma, Doria Cluyticefolia, Oligothrix 

 gracilis, and some species of Senecio. Spiral 

 cells also occur on the seed of Hydrocharis. 

 The best way to observe the elastically ex- 

 panding hairs is to place a thin slice of the 



