1906.] 



The Spread of Fungus Diseases. 



263 



on the other hand, infected plants are more robust and vigorous 

 than uninfected ones, suggesting that the amount of food 

 required by the parasite stimulated the host-plant to exercise 

 greater activity in the matter of obtaining food. However, 

 when the plant has reached the flowering stage, the fungus 

 present in its tissues assumes an aggressive attitude, its 

 mycelium enters the young ovaries and in place of seed pro- 

 duces its black, powdery mass of spores. 



In many instances the relation between parasite and host- 

 plant has become so thoroughly adjusted that no injury is 

 experienced by either ; on the other hand, mutual benefit is 

 derived from the combination. Such a condition of things is 

 termed symbiosis, a remarkable instance of which has been 

 described by Freeman as existing between fungi and three 

 kinds of rye-grass respectively, Lolimn teimtlentinn^ L,, L. 

 perenne, L., and L. italimvi, A. Br. The life-history has been 

 followed in L. te)nulentuin or darnel, and briefly is as follows : — 

 The mycelium of the fungus is present in the seed on ger- 

 mination ; this mycelium commences growth and keeps pace 

 with the host-plant, finally again entering the seed, where it 

 remains in a resting condition until germination takes place, 

 when the same round is repeated. A series of experi- 

 ments showed clearly that infected plants were more vigorous 

 than uninfected ones. So certain is the fungus of perpetuating 

 itself by this vegetative method without ever leaving the host- 

 plant, that the production of spores has been completely 

 arrested, hence the affinities of the parasite cannot be deter- 

 mined. 



In the absence of spores no other plants of the same kind can 

 be infected, consequently there are two races of each of the 

 three rye-grasses, one infected or in symbiotic connection with 

 a fungus, and one race not, and without the possibility of 

 becoming so. Microscopical examination of a commercial 

 sample of the seed of darnel, showed that over 80 per cent, 

 were infected. 



Many other instances could be given where disease is 

 perpetuated from one generation of plants to another, due to 

 the presence of hybernating mycelium, but perhaps sufficient 

 has been stated to prove that this subtle method of transmitting 



