11 



however, pass through a much more complex life-cycle, during which 

 a given fungus may produce several kinds of spores and assume 

 several forms so unlike each other that they can be recounized as 

 different stages of the same plant only by careful, patient cultivation 

 and study. It is convenient to select some one stage of such a varia- 

 ble fungus us its perfect or adult form, and it is natural and logical 

 to regard as such that stage in which the fungus shows the greatest 

 elaboration of structure, while the simpler stages through which it 

 passes are commonly called imperfect form,s. This tendency of fungi 

 to variety in form, or pleomoi-phism, as it is called, greatly increases 

 the difficulty of their study and complicates those problems which 

 concern the successful combating of fungous diseases. 



A question which very naturall}' suggests itself is : Why do fungi 

 attack and cause diseases of other plants, instead of living independ- 

 ly? This question involves matters of the greatest interest and of 

 fundamental importance and significance. It is well known that all 

 gi'een plants owe their characteristic color to the presence of a defin- 

 ite pigment known as leaf-green or chlorophyll, which is so generallv 

 present among the higher [)lants, that to most minds the very word 

 plant carries with it the idea of greenness. Now the possession of 

 chlorophyll is the preeminent feature which gives to plants their all- 

 important place in the economy of nature. No living thing can con- 

 tinue to live on inorganic substances, but all require as food some 

 of those materials of comparatively complex chemical composition, 

 known as organic substances. The materials furnished by the earth, 

 the air and water are all of simple composition and unorganized, but 

 in leaf-green we have the connecting link, the means of bridging the 

 interval between the inorganic and the organic. We need not here 

 discuss the process in detail. It is sufficient for our present purpose 

 to say that in Nature's laboratory of the leaf, some of the simple con- 

 stituents of air and water are combined, by the action of leaf-green in 

 the sunlight, into the complex organic compounds which serve the 

 plant as food. The chemistry of this remarkable process is not well 

 understood, but the commonest permanent form in which these 

 food materials appear is that of starch. 



Now, as was noticed above, the threads of the fungi are white, 

 uncolored ; that is, they contain no leaf-green. Consequently, the 

 fungi cannot elaborate their own food material, but must obtain it 

 ready elaborated, from some other source. Evidently the available 

 sources of organic food-supply fall under two heads, living organisms^ 



