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)Welopment of plants 



— 205 



ferment that dissolves the cell walls and thus opens the way for 

 the entrance of the pest. The cultivation of plants has doubtless 

 weakened their resistance in many cases to the attack of the para- 

 sites, and like ourselves the plant may inherit a weaker constitu- 

 tion that is more subject to disease. Nearly every state now em- 

 ploys 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). Whatever 



Fig. 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. 



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

 sist of filaments of delicate cells or tubular growths, called hyphae 

 (sing, hypha). These fine filaments or tubes are the essential 

 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 structure 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. Even in the larger fungi, as the mushroom, 



