380 THE FUNGI WHICH CAUSE PLANT DISEASE 



life is carefully studied. The chief of these is the incapacity of a 

 race to attack the host-plants of another race. Such forms as 

 are thus outwardly identical but which show a constant difference 

 in their mode of life are known as "biological" species or asformoe 

 speciales. 



"A specialized form is considered to be 'sharply fixed' or 'not 

 sharply fixed' according as it is wholly incapable, or sometimes 

 capable, of attacking the host-plants of the other specialized forms 

 of the same fungus. Thus the P. graminis of wheat (P. graminis 

 f. sp. Tritici) is not sharply fixed, for it can attack barley, rye, 

 «&c., sometimes. The P- graminis foimd on grasses of the genus 

 Agrostis {P. graminis f. sp. Agrostis) is sharply fixed, for it attacks 

 this genus only and does not pass to the other grasses on which 

 it has been tried. 



"But even the not sharply fixed forms, such as the P. graminis 

 of wheat, may be entirely incapable of attacking some of the 

 species which bear other forms of the same fungus. In other 

 words a form may be not sharply fixed in regard to some host- 

 plants and sharply fixed in regard to others. A striking instance 

 of this occurs in India. P. graminis can be divided amongst others 

 into races on wheat (/. sp. Tritici), rye and barley (/. sp. Secalis), 

 and oats (f. sp. Avence). The/, sp. Tritici can attack barley some- 

 times, and did so in four out of sixteen of our inoculations, but it 

 does not, in India at least, attack oats. Hence it is sharply fixed 

 in regard to oats and not sharply fixed in regard to barley. The 

 /. sp. Secalis on barley also does not pass to oats, but infected 

 wheat doubtfully in two out of sixteen inoculations. These two 

 forms are common in India, and the practical bearing of their 

 not passing to oats is considerable, for the /. sp. Averm has not 

 yet been observed in this country." 



The mycelium branches intercellularly and bears small haus- 

 toria which penetrate the cells. In the barberry it is local. The 

 epiphyllous pycnia appear first followed soon by the mainly 

 hypophyllous secia. The flask-shaped pycnia at maturity bear 

 numerous pycniospores and exserted paraphyses. Their hyphae 

 are orange-tinted, due to a coloring matter in the protoplasm or 

 later in the cell walls. 



The aecium originates in the lower region of the mesophyll 



