DEVELOPMENT OF PLANTS 217 



cultivation of plants has doubtless weakened their resistance in 

 many cases to the attack of the parasites, and like ourselves the 

 plant may inherit a weaker constitution that is more subject to 

 disease. Nearly every state now employs experts to study the 

 diseases caused by these fungi and to devise means for killing 

 them and to develop more resistant plants. 



The fungi are of very simple structure because they live upon 

 foods already manufactured for them. There is no longer a 

 necessity for a plant body that is complex, because each part is 

 adapted to the performance of one or another of the many func- 

 tions that cooperate in the construction and distribution of the 

 organic substances. Consequently, the fungi do not exhibit 

 many of the characteristics of chlorophyll-bearing plants. Their 

 cell walls are thin and inclose a watery, colorless protoplasm in 

 which are usually dispersed many small nuclei (Fig. 129). What- 



FlG. 129. A few cells from a branching filament of green mould, Peni- 

 cillium, showing the granular character of the cytoplasms and the absence 

 of plastids. The colorless areas in the cells, vacuoles, contain principally 

 water. 



ever form the plant body may assume, it will usually be found to 

 consist of filaments of delicate cells or tubular growths, called 

 hyphae (sing, hypha) . These fine filaments or tubes are the essen- 

 tial portion of any fungus and as they spread over the substance 

 upon which they feed they form a branching and interwoven 

 mass of threads collectively known as the mycelium. This struc- 

 ture is well illustrated in the hyphae that spread over bread and 

 fruits, forming a cobwebby mass or mycelium that is commonly 

 known as mould or mildew. Bodies of various forms arise from 

 the mycelium which are often mistaken for the fungus itself, as 

 the mushroom, but this mushroom sustains the same relation to 

 15 



