50 MICROSCOPIC ruxGi. 



mycelinm, penetrate the intercellular spaces, and 

 insinuate tliemselves in a complete network, amongst 

 the ceils of which the leaf, or other diseased portion 

 of the plant, is composed. High powers of the 

 microscope, and equally high powers of patience 

 and perseverance, are necessary to make out this 

 part of the structure. We may regard the whole 

 mycelium of one pustule, or spore-spot, as the 

 vegetative system of one fungal plant. At first 

 this mycelium might have originated in a number 

 of individuals, which afterwards became confluent 

 and combined into one for the production of fruit, 

 that is to say, an indefinite number of points in the 

 vicinity of the future mycelium developed threads ; 

 and these, in the process of growth, interlaced each 

 other, and ultimately, by means of transverse pro- 

 cesses, became united into one vegetative system, 

 in which the individuality of each of the elementary 

 threads became absorbed, and by one combined 

 efibrt a spore- spot, or cluster of fruit, was produced. 

 In the first instance a number of minute, trans- 

 parent, colourless cellules are developed from the 

 mycelium : these enlarge, become filled with an 

 orange-coloured endochrome, and appear beneath 

 the cuticle of the leaf as yellowish spots. As a 

 conseq\ience of this increase in bulk, the cuticle 

 becomes distended in the form of a pustule over the 

 yellow cell ales, and at length, unable longer to 

 withstand the pressure from beneath, ruptures in 

 irregular, more or less elongated fissures (plate YII. 



