DISEASES OF CULTIVATED PLANTS 435 



sponse to resistance to parasitic attack ; thus it happens that the po- 

 tato scab organism induces the growth of cork cells on the outside of 

 the potato and makes a roughness. The roughness is not the scab 

 fungus but the corky growth of the tubers in response to the scab 

 attack. In a similar manner the attack of the scab fungus upon the 

 apple results in the roughening of the apple skin through the devel- 

 opment of more protective or wound cork. The most remarkable ex- 

 ample of this multiplication of protecting or outer cells in response 

 to the attacks of parasitic fungi is found in leaf-curl of the peach and 

 in the pockets or bladders of the plum, where we have such a rapid 

 multiplication of cells in response to the stimulus of the fungus as to 

 bring about an entire transformation in the form and structure of the 

 parts. While we may think of this abnormal development as the re- 

 sult of fungous growth, it is only indirectly so. It is in fact a re- 

 sponse of the host to the stimulus of the invading fungus. The 

 nature of the stimulus or excitation exerted by particular parasitic 

 fungi is a highly interesting subject for investigation. 



CONCERNING PARASITIC FUNGI. 



A fungus (plural, fungi) is a plant, a member of the class 

 called fungi. The fungi are low in the scale of plant life, being 

 classed with the algae and other similar plant forms. They are lower 

 still in the life scale than the mosses and liverworts ; above the mosses 

 come the fern-plants, and above these the seed plants, such as grasses 

 grains, clovers, trees, shrubs, herbs and the like, with which we 

 come in contact every day. The fungi are distinguished from higher 

 plants as well as from their nearer relatives, the algae, by the absence 

 of green color, and for that reason, we may assume, by the lack of 

 power to prepare their own food from the mineral substances dis- 

 solved in water, and from the gases contained in the atmosphere. 

 Herein they are marked off from most groups of plants: the fungi 

 must live upon the substance of living or dead plants or animals. If 

 they ever possessed the power of utilizing the same foods as most 

 other plants, this ability has been lost. Parasitism is usually taken 

 to indicate degeneracy in character. One way of regarding the fungi 

 is as algse without chlorophyll, to which the latter owe their green 

 color. As above stated, the fungi are, in the absence of chlorophyll, 

 forced to live upon the dead remains of plants or animals, or to prey 

 upon the living organisms. 



CLASSES OF FUNGI. 



Such fungi as subsist upon living plants or animals are called 

 parasitic fungi. A parasite is one who eats at another's table and the 

 adjective parasitic comes from this word, parasite. It is the parasitic 

 fungi especially of which we must learn, since this class produce 

 diseases when they attack other plants. The plant attacked is the 

 host plant, however unwilling the entertainment of the sycophant. 



Most fungi are very minute in size and require the use of a 

 microscope to study their parts; certain ones, however, such as the 

 mold upon bread or other foods, may be seen very easily to consist 

 of fine, thread-like growths interwoven together, and bearing certain 

 rounded parts upon erect branches. Some idea of fungus-structure 



