104 HAIRS 



number of unicellular rays which are joined together at their 

 base but free at their tips. 



Certain climbers are materially aided in clinging to their 

 support by the possession of stiff hairs. Thus the ridges on the 

 stem of the Goosegrass (Galium aparine) bear numerous reflexed 

 unicellular hairs, shaped like a hook and having a stout base 

 and a very strongly thickened tip. In the Hop the ridges are 

 similarly beset by stiff two-armed hairs seated on a small elevation 

 of the epidermis ; the two arms are situated in the vertical plane, 

 and the downwardly directed one is considerably longer than 

 the other. Extreme types of multicellular climbing hairs are 

 furnished by the prickles of the Rose and Bramble. 



Whilst the walls of the ordinary covering hairs are generally 

 not appreciably thickened, both branched and unbranched hairs 

 may possess thick walls which are frequently silicified or calcined ; 

 when thus stiffened, they constitute a chevaux-de-frise against 

 small animals (e.g. slugs). Good examples are afforded by the 

 unbranched bristle-hairs of many Boraginaceae (e.g. Borage, 

 Comfrey, etc.) and the branched types found on the Stocks and 

 other Cruciferse. Their effect is often accentuated by the pres- 

 ence of numerous minute teeth on their surface (Fig. 48, A). 



One of the most striking examples of hairs acting as a de- 

 terrent to animal attacks is, however, furnished by the uni- 

 cellular stinging hairs of the Nettle (Urtica) (Fig. 50, D). Each 

 is borne on a multicellular stalk in which is embedded the thin- 

 walled swollen base of the actual stinging hair. The upper part 

 of the latter is comparatively thick-walled and tapers gradually 

 to near the apex, where it suddenly enlarges to form a tiny 

 bead-like tip (Fig. 50, E). The lower part of the wall is calcined, 

 the upper part silicified. The living protoplasmic contents often 

 show distinct streaming movements, and include a large vacuole 

 filled with acrid sap. When an animal brushes against one of 

 these hairs, the little tip breaks off, leaving exposed a fine needle- 

 like point formed by the upper tapering part of the hair. As a 

 result of the pressure of contact, this fine tube penetrates the 

 skin, and the compression of the bladder-like base injects the 

 contained fluid into the wound. 



The hairs of many plants produce secretions J which are often 

 * Water- and sugar-secreting hairs are considered in Chapter XII. 



