76 STINGS, PRICKLES. AIR-CHAMBERS. 



would have otherwise formed a larger cuticle had taken the shape 

 of hairs : whilst in damp shady places, which favour the extension 

 of the leaves and stems, their surface is quite smooth, all the 

 material being then required to form cuticle. It will be shown 

 hereafter (. 278) that the development of hairs on the surface 

 of the leaves of plants growing in dry situations, favours the 

 absorption of moisture. 



97. Sometimes the hairs are tubular and pointed, and are 

 fixed upon minute glands in the cuticle, which secrete an acrid 

 fluid ; and if but very slightly touched, the reservoir at the base 

 is compressed, and the fluid forced up through the tube, into the 

 wound made by its pointed extremity. Such hairs are termed 

 stings ; and the Nettle affords a familiar example of them. The 

 prickles of the Rose and other shrubs are also appendages of the 

 cuticle, with which they are stripped off, and from which it is easy 

 to detach them. They are thus distinguished from thorns, which 

 proceed from the wood of the branch, and which, as will be here- 

 after stated, may be regarded as stunted leaf-buds. Prickles, 

 after being once formed, and hardened by the process already 

 described (. 74), undergo no subsequent enlargement ; and, 

 accordingly, if the cuticle on which they are fixed should be 

 extended, their base is not able to expand in the same propor- 

 tion, and they drop off, leaving scars on the surface of the stalk. 



98. Another interesting modification of cellular tissue, is that 



which surrounds the spaces or 

 cavities, formed in certain plants 

 for special purposes. Thus in the 

 Duckweed, the leaves are pro- 

 vided with a set of air-chambers, 

 which give them great buoy- 

 ancy ; and nothing can be more 

 beautiful than the manner in 

 which the walls of these cham- 

 bers are built up of muriform 

 cellular tissue. In other cases, 

 t!lese -vifeappear to be formed 

 CELLULAR TISSUE as receptacles for certain secreted 



<r, a, horizontal partitions. i , , ., 



products : and here, too, they 



