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found only in early spring, others that occur only during the warmer 

 part of the summer, and still others that grow only in autumn, while 

 there are some that occur throughout the growing season. Also, there 

 are many species that are found (jnly in the warm countries of the 

 tropics, while others occur only in temperate or colder regions. How- 

 ever, it is very difficult to say whether these differences are due pri- 

 marily to heat or not. There are so many causes acting on plants at 

 the same time that it is often impossihle to single them out without 

 performing control-experiments, and we are very apt to ascribe cer- 

 tain effects to heat when they really are due primarily to other causes. 

 The differences between spring and summer or spring and autumn 

 mushrooms are probably not due primarily to temperature, though the 

 differences between tropical and cold-climate species may be. 



Light. — Light is not nearly so important to mushrooms as it is to 

 green plants. In fact most fungi grow better in the dark than in the 

 light. There are, however, many species that grow only in the shade 

 of other plants, while others grow only in open sunny places. The 

 difference in the amount of light is probably not the only reason for 

 this, but, in some cases, it may be the principal one. Many kinds of 

 mushrooms, too, grow very well in the dark but cannot produce per- 

 fect fruit-bodies unless they have light. The fact that fungi can grow 

 in the dark makes it possible for them to flourish in places where no 

 other plants can exist. In underground caves, mines, etc., certain 

 kinds of fungi are practically the only plant life. 



Suhstratuui. — The material on which a fungus grows, whether it 

 be soil, wood. bark, dead leaves, or other substance, is spoken of as 

 the substratum. There is scarcely anv kind of substratum that is not 

 suitable for some kinds of fungi, but many of the mushrooms arc 

 limited to very definite kinds of substratum. There are a number of 

 species, for instance, that grow only on dung; others that are found 

 only on leaf-mold or rich humus in woods ; while still others prefer 

 the soil of pastures, lawns, etc. There are many kinds, too, that oc- 

 cur only on wood, and, of these some are cjuite cosmopolitan and 

 grow on various kinds of wood, while others are found only on the 

 wood of a particular kind of tree. A number of mushrooms spend 

 their lives as parasites on other living plants, but these will be spoken 

 of again later. 



Water. — As is the case with nearlv all ])lants, water is one of the 

 most important factors affecting the life of mushrooms. Plants are 

 often separated into three groups based on the relative amount of water 

 necessary for their successful growth. Those plants that can g-et along 

 with a very small amount of water are called xerophytes, while those 

 that require a very large amount are called hydrophytes, and those 



