20 STRUCTURE AND PROPERTIES 



being the supporters of separate little glands, and 

 the channels by which the fluid secreted by a 

 gland passes off. It is worthy of remark, that in 

 all glands furnished with excretory hairs, the 

 juice secreted is of an acrid nature, and is only 

 directed towards the exit prepared for it, when 

 the gland, pressed on by some exterior force, 

 suffers the fluid to escape; the juice then flows 

 through the excretory canal, which by its pointed 

 extremity punctures the skin of the animal which 

 has incautiously touched the plant, and deposits 

 its fluid beneath it. This defensive organization 

 closely resembles the structure of the venom bag 

 and tooth of serpents, and is well illustrated in 

 the common nettle. Lymphatic hairs are much 

 more abundant than the preceding ; they are of 

 very various forms, and are only found on those 

 parts of plants which are exposed to the air. 

 Their office is probably that of preventing evapo- 

 ration in certain portions of the plant, and of pro- 

 tecting the more delicate organs against cold, 

 moisture, insects, &c. ; and in support of this view 

 of their use, it will be found that the tender bud is 

 often defended by these hairs, which, when the 

 shoot approaches to maturity, either drop off en- 

 tirely, or become thin and widely scattered. 

 12. Air Cavities. The cellular tissue is often 



