WHITE RUSTS. 125 



these spots appear, the plant is more or less de- 

 formed, swollen, or blistered, even before the para-, 

 site makes its appearance at the surface. These 

 white pustules have a vegetative system of ramify- 

 ing threads which traverse the internal portion of 

 the plants on which they are found : these threads 

 constitute what is termed the mycelium. Not only 

 when the plant is deformed and swollen with its 

 undeveloped parasite do we meet with the threads 

 of mycelium in its internal structure, but also in 

 apparently healthy portions of the plant, far re- 

 moved from the evidently infected spots. These 

 threads are unequal in thickness, much branched, 

 and often with thick gelatinous walls filled with a 

 colourless fluid. They creep insidiously along the 

 intercellular passages, and are provided with cer- 

 tain appendages in the form of straight thread-like 

 tubes, swollen at their tips into globular vesicles 

 (plate X. fig. 204). These threads do not exceed 

 in length the diameter of the mycelium which bears 

 them. The appendages communicate in their in- 

 terior with the mycelium, and contain within them 

 the same fluid, which at length becomes more 

 watery, and the terminal vesicles have their walls 

 thickened, so as to resemble, on a casual observa- 

 tion, granules of starch. Dr. de Bary conceives 

 that these appendages serve a similar purpose to 

 the tendrils or suckers of climbing phanerogamic 

 plants ; i.e., to fix the mycelium to the cells which 

 are to supply the parasite with nourishment, A 8 



