154 FORMS OF HAIRS. [BOOK r. 



conical, cylindrical, or moniliform, thickened slightly at the 

 articulations (torulose), as in Lamium album, or much en- 

 larged at the same point (nodulose), as in the calyx of Achy- 

 ranthes lappacea. In Polystachya luteola the hairs of the 

 labellum are moniliform, or necklace-shaped, with the articu- 

 lations 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. 



The Rev. J. B. Reade has described a peculiar form of hair 

 in the common Mustard. " If a seed be immersed in water, 

 the testa, in the course of a few hours, will be covered with 

 very minute 'vessels/ starting like radii from its surface. 

 The peculiar refractive power of these ' vessels' renders them 

 a remarkably difficult microscopic object. Their form is en- 

 tirely novel. A number of wine-glasses, with long stems and 

 inserted into each other, may furnish a somewhat apt illustra- 

 tion of their remarkable appearance; and as the walls of the 

 bell-shaped portion are strengthened by a spiral fibre, the 

 { vessels' may be described in one word as fibro-campanulate." 



Hairs are sometimes said to be fixed by their middle (Plate 

 I. fig. 10. c) ; a remarkable structure, common to many dif- 

 ferent 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 cell of the 

 epidermis above the level of the rest, and the subsequent 

 development of a simple hair from its two opposite sides. 

 Such would be more correctly named divaricating hairs. 

 When the central cell has an unusual size, as in Malpighia, 

 these hairs are called poils en navette (pili Malpighiacei) by 

 De Candolle, and when the central cell is not very apparent, 

 poils enfausse navette (pili pseudo-Malpighiacei, biacuminati) , 

 as in Indigofera, Astragalus asper, &c. In many plants the 

 hairs grow in clusters, as in Mallowworts (Malvaceae), and 

 are occasionally united at their base : such are called stellate, 

 and are frequently peculiar to certain natural orders. (Plate 

 I. fig. 10. a.) 



If highly magnified, the stellate hairs of Deutzia corymbosa 



