EXCRESCENCES. 21 5 



and over all may be a well-marked epidermis, 

 with stomata, or cork with lenticels. 



The chief seat of these hypertrophies and 

 what is more remarkable development of new 

 tissue elements not found elsewhere in the leaves, 

 or even in the species, is the mesophyll, and 

 various speculations and hypothesis have been 

 founded on these curious phenomena. 



Erineum. The simplest excrescences on plants 

 are certain hair-like developments of epidermal 

 cells due to the irritation of species of PJiytoptus, 

 and similar insects which rise in clusters on the 

 surfaces of leaves and by their colours, consistence, 

 arrangement in patches, spots, etc., so simulate 

 fungi that Persoon was deceived by them and 

 gave them the genus name Erineuvi. They occur 

 on most of our trees, e.g. Poplar, Lime, Oak, and 

 are very common in the Tropics. Usually pale 

 or even white at first, they turn brown as the 

 hair-like outgrowths die and lose their sap, but 

 since the latter may be bright coloured yellow, 

 red, purple, the patches are sometimes very 

 conspicuous objects on smooth leaves. 



In many cases these hairs exactly resemble 

 in shape and other characters the abnormal root- 

 hairs found on roots exposed to the effects of 

 prisonous reagents, or of unsuitable food-materials, 

 or the rhizoids developed from wounded Algae, 

 etc. 



Intumescences are similar trichomatous out- 

 growths not associated with insects or fungi, 

 and due to some disturbance of the balance 



