CHAP. III. FUNCTION OF NUTRITION. 221 



a class of substances which are excreted from plants by 

 various glands seated on the surface of their stems, 

 leaves, and other organs. Many of them are of the 

 same description as those which are formed within 

 the plant by internal secretions, such as acids, oils, &c. ; 

 but some of them are peculiar. They may be con- 

 sidered as more strictly analogous to the various ex- 

 crementitious matters ejected by animals than those of 

 the former class; and the glands by which they are 

 formed are for the most part more complex and 

 better defined than those which are seated in the 

 interior of plants. The external glands (see art. 31. 

 andyz^. 20.) by which these matters are excreted often 

 form a sort of clammy pubescence upon the epidermis. 

 They frequently resemble hairs tipped with a little 

 globular mass by which the excreted matter is more 

 especially elaborated. 



(213.) Fraxinella. The common Fraxinella is 

 covered with minute glands which excrete a volatile 

 oil. This is continually evaporating from its surface, 

 and on a calm still evening forms a highly inflammable 

 atmosphere round the plant. If a candle be brought 

 near it, the plant is enveloped by a transient flame 

 without sustaining any injury from the experiment. 



(214.) Stings, The stinging plants prepare a 

 caustic juice which is contained in a cellular bag sur- 

 mounted by a hollow bristle. When the bristle is 

 gently pressed the fluid is forced through it and flows 

 out at the summit through a minute orifice, as we have 

 stated (art. 31. and^.20. a). If the bristle enters a pore 

 of the skin, the caustic fluid is introduced and produces 

 the painful sensations familiar to all who have ever handled 

 a common nettle. The Loaste have stings which give 

 a still more irritating sensation than the nettles. The 

 Malpighise are furnished with a multitude of doubly 

 pointed bristles which lie parallel to the surface of their 

 leaves, to which they are attached by a short hollow stem. 

 These contain a slightly caustic fluid. 



(215.) Glue. The gummy excretions on the stems of 



