MICROSCOPIC OBJECTS. 18 



Fig. 35. Stings from Stinging -nettle (Urtica dioica), x 40. 



Stings of plants may be regarded as 'enlarged forms of 

 hairs, having a bulbous base, in which the acrid or irritating 

 fluid is contained. They are chiefly found in the N. O. Urti- 

 cacese. The mode of action of the sting is as follows : 

 Directly the point of the sting is broken off, a fine tube is 

 left exposed ; and the sting being at the same time slightly 

 pressed upon its bulbous base, the acrid fluid rises through 

 this tube and enters the flesh at the point made by the punc- 

 ture of the sting. 



These stings may be mounted dry or in liquid. 



Fig. 36. Petal of Scarlet Pimpernel (Anagallis arvensis) , 



x200. 



This petal has been drawn to illustrate the spiral- vessels 

 which run up the veins of the petals of many flowers. Another 

 example is shown at fig. 38. Many of the Composite florets 

 show a similar structure well when mounted in fluid or bal- 

 sam ; the florets of Senecio Jacobaa form a case in point. 



Transverse sections may be taken of some of the thicker 

 petals, as they often show the mamilla in its natural position 

 better than in a semiopaque view. 



This petal is best mounted in balsam. 



