THE FUNGI WHICH CAUSE PLANT DISEASE 333 



phologically inseparable as are also their secia when grown upon 

 their common host. 



An excellent example of such biologic specialization is offered 

 in the common pine Peridermium. iEcia may be produced upon 

 the pine by sowing of Coleosporium teliospores from Senecio, 

 Campanula, Pulsatilla, etc., but the seciospores which develop 

 on the pine are capable of in- 

 fecting only those species of 

 hosts from which the teho- 

 spores were taken. 



Similarly Eriksson "* has de- 

 termined that though rusts 

 from many grains can infect 

 the barberry, the seciospores 

 there produced are not capable 

 of infecting plants of species 

 other than those from which 

 the fungus was derived, or at 

 most they can infect but a 

 very limited number of species. 



A further complication arises 

 from the facts obtained through 

 experiments in various coun- 

 tries, which have shown that what is apparently the same species 

 may consist of a large number of strains or varieties which be- 

 have differently in different geographic areas. The stem rusts of 

 wheat and barley, for instance, are very similar, interchanging 

 hosts easily and being capable of transfer to various grasses in this 

 country, though in Sweden the stem rust of wheat goes with 

 difficulty to barley and rye, while the stem rusts of barley and 

 rye interchange hosts very easily. 



Owing to the prominence of its author and its place in litera- 

 ture a word may be given to the usually discredited mycoplasm 

 theory ^^^^^- ^i^ gf Eriksson. This affirms the existence in the 

 cells of wheat grain of an intimate mixture of rust protoplasm and 

 host protoplasm. This mycoplasm may rest thus for months. 

 Finally the host-cell nucleus becomes digested and the fungous 

 plasm develops to a mycelium which proceeds to invade the sur- 



Fio. 249. — Urediniospores in Rubus show- 

 ing nuclear conditions. After Black- 

 man. 



