96 



VII. TRICHOMES WAX". 



water. If the hair is already dead, air will be found in its 

 interior, and its apex is then no longer intact. An uninjured hair 

 presents the appearance represented, iri Fig. 36. The hair is 

 unicellular, sharply tapering, swollen a its apex into a small 

 knob. At the base the hair broadens out, and the bulb thus 

 formed is sunk in a cup which is developed from the tissue of 

 the leaf. As jts developmental history 

 (ontogeny) shows, this hair springs from 

 a single epidermal cell, lying at the same 

 level with its neighbours ; afterwards the 

 strongly swelling foot of the hair is lifted 

 up on a column of tissue, which is covered 

 by the epidermis, and is formed internally 

 of hypodermal (sub-epidermal) tissue. 1 In 

 the hair itself is to be seen streaming of 

 the protoplasm. The nucleus is usually to 

 b'e seen within the bulb, suspended by 

 protoplasmic threads. The cuticle shows 

 oblique striae, which ascend in the same 

 direction in all the hairs. The wall of the 

 terminal knob, and the parts of the hair in 

 its neighbourhood, are silicified through 

 their entire thickness. This can be proved 

 by treatment with concentrated sulphuric 

 acid, and subsequent addition of 20 per 

 cent, chromic acid, by which means the 

 organic membrane is completely removed 

 and a siliceous skeleton left. Lower down 

 the thickness of the silicified portion rapidly 

 diminishes, and ultimately is limited to the 

 cuticle. As the ebullition upon treatment 



FIG. 36Stinging hair of f tne na< i r w ^h hydrochloric acid shows, 

 Urtica dioica together with tne unsilicified portions of the wall are, on 

 a fragment of the epidermis, r 



on which is a small bristle the other hand, impregnated with carbon- 

 ate of lime, by which the rigidity of the 



hair as a whole is enhanced. As already noted, hairs are often 

 found with their points broken off. The breaking-off of the knob 

 is facilitated by the wall of the hair just under it having a thin 

 part. The oblique insertion of the knob, on the other hand, 



1 That is, the hair is borne upon an emergence. [ED.] 



