PART VII. 



FUNGI, CONSIDERED CHIEFLY IN RE- 

 LATION TO SOME COMMON DISEASES 

 OF PLANTS. 



CHAPTER XLVI. 



FUNGI: GENERAL. 



I. Many plants are rendered unhealthy through inadequate 

 supply of air to their roots, excessive dampness, or great 

 dryness of the soil, too high or too low a temperature of the 

 surrounding atmosphere, and other similar unsuitable conditions 

 of soil and climate. Insects are also responsible for the de- 

 struction of numbers of plants, but perhaps the most extensive 

 and insidious diseases of farm and garden crops are due to 

 the attacks of a class of lowly-organised plants known as fungi. 



2. The fungi, of which more than 40,000 species have been 

 described, constitute a sub-division of the Thallophytes (see 

 p. 322), and are all characterised by a complete absence of 

 chlorophyll. 



3. Hypha and Mycelimn. — The body of a fungus is composed 

 of long, thin filaments termed hyphm. Each hypha is a trans- 

 parent, tube-like structure, the wall of which usually contains 

 a larger or smaller amount of chitin, a substance commonly 

 met with in the animal kingdom ; only in a few instances is 

 cellulose present in the cell-membranes of fungi. 



Lining the hyphse or filling them is a colourless cytoplasm, 

 in which are numerous small nuclei and often many large 



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