OTHER SAC FUNGI 



253 



The caterpillar and grub fungi. These extraordinary parasites 

 (Cordyceps) grow in the bodies of certain caterpillars and other 

 larvae, and in their pupae. The body cavity of 

 the insect becomes filled with the mycelium, 

 and generally mummified, after which a long- 

 stalked sac fruit grows out from between cer- 

 tain segments (Fig. 225). 



The truffles. The truffles are very remark- 

 able sac fruits, sometimes as large as pota- 

 toes, which are developed on my- 

 celium that is generally associated 

 with the roots of certain trees. The 

 commonest truffle on the market 

 (Tuber Irumale) comes from the re- 

 gion of Perigord, in central France, 

 and is the most prized of all the edi- 

 ble fungi. It grows under certain 

 kinds of oak trees, and is found by 

 dogs and swine that are trained to 

 discover its location, and which detect 

 the fungus by a characteristic odor. 

 The association of the mycelium of 

 the truffle with the roots of 

 the oak tree is an excellent 

 example of what is called 

 a mycorrhiza, and is dis- 



A, ergot grains on a 



head of barley; 13, CUSSed in Sec. 278. 

 small sac fruits (as- ^ t j ^ t Flo . 2 25 



cocarps) developing r J y 



from an ergot grain. The Spot diseases of plants Caterpillar fungus 



larva of the May 

 beetle, which lives 



underround 



upon the leaves and fruit. Many of them are 



caused by sac fungi, as the strawberry-leaf spot 



(Sphcerella), black spots on grasses and clover 



(Phyllachora) resembling rust spots, tar spots on willow and 



maple (Rhytisma), and the apple scab (Venturia). Some of the 



