EXCRESCENCES. 219 



solid except for the inner cavity containing the 

 eggs -Neurotus, Cyntps, Honnomyia, etc. These 

 are comparable in general characters to the 

 nodules on roots. 



Fungus galls with similar external features when 

 young are found on Maize {Ustzlago Maydis\ 

 and betray their nature by the black powdery 

 spores as they mature. 



Bud galls on Willows are due to Cecidomyia, 

 which causes several internodes to swell out into a 

 greenish barrel-shaped mass, from which leaves 

 may spring. 



Small irregular excrescences on Willow stems 

 are referred to Pkytoptus, and another species of 

 the same insect induces similar swellings on Pines 

 which are not surcharged with resin. 



American Blight, or Woolly Aphis, on Apples 

 especially, causes the tumour-like swellings covered 

 with sticky white fluff, which is a waxy excretion 

 of the insect. Galls on Pilea, in Java, are due to 

 an Alga PhytopJiysa. 



Root-nodules or nodosities are frequently caused 

 by insects e.g. Centhorhynchtis, a beetle which 

 attacks Crucificers, Cynips and allied " gallflies " 

 of Oaks, and the notorious Phylloxera. But similar 

 root-galls are produced by Nematode worms, 

 Heterodora, on Beets, Tomatoes, Cucumbers and 

 numerous other plants, and by the Slime fungus 

 Plasmodiophora, and it is not always easy to 

 distinguish such cases from the fungus-galls {Myco- 

 cecidia) on the roots of Aiders, Juncus, and Legumi- 

 noseae where the symbiosis of bacteria or fungi with 



