128 



Thus we believe that the red coloring matter may be looked upon as a 

 necessary reaction of the cell to the influence of different factors connected 

 with a relatively over-abundant supply of light. One of these factors may 

 be the lowering of the temperature due to a change in the latitude or longi- 

 tude of the place of growth. 



If we look back to the many changes undergone by the plants in their 

 morphological and chemical structure because of any change in latitude of 

 the place of growth, we cannot shut our eyes to the conviction, that not in- 

 frequently in these changes of place may be sought the reason for a predis- 

 position toward disease or, on the other hand, toward greater immunity. 



We have mentioned that the western squarehead wheat grown in 

 eastern regions has greater susceptibility to frost and now remind the 

 reader that parasitic diseases may also be dependent on the different mode 

 of development of the host plant inherited in the seed. One should con- 

 sider, for example, the fact that many parasitic fungi appear or are especial- 

 ly abundant at definite periods. In case such fungi only attack young leaves, 

 the presence of young leaves when the spores are ripening will determine an 

 epidemic. The rapidity with which a plant passes through its develop- 

 mental cycle in any given climate is a determining factor in this question. 

 If it develops slowly, its leaves are young and remain susceptible for a longer 

 time, giving a greater danger of fungus infection. If a variety matures 

 quickly (for example, one introduced from more northern or eastern 

 regions) then the leaf may be fully matured at the time of the actual distri- 

 bution of the spores and therefore be resistent to many parasites. 



Such circumstances deserve greater consideration than has been given 

 them as yet. They will also be a factor in the discussion of the "biological 

 races" of individual parasites, for it is most probable that often infections 

 of the most closely related host species fail because the host plant at the 

 time of infection is already in an advanced developmental stage, in wliich 

 the leaf is more mature, i. e., has thicker walls and less cell-content. The 

 fact that the fungus infection is connected with a definite developmental 

 stage of the host plant is shown, for example, in the rust fungi of grains. 

 Eriksson^ states that the rust occurs earlier in the varieties ripening early 

 and recent observations show that the different forms of Puccinia have defi- 

 nite periods for attacking grain. Thus it was shown in 1904- that Puccinia 

 gluiiuirmn a[)i)eared first and foremost in wheat, then followed P. dispersa 

 which, however, attacked only those organs and varieties which were still 

 immature. Later, slowly ripening varieties of wheat were found badly at- 

 tacked by P. dispera and slightly by P. glumarum, wdiile the converse is 

 true for varieties maturing early. P. graminis was found in stored grain. 



semence 



2 Jahre 

 Getroiderost 



sson, J., Sur I'origine et la propagation de la rouille des cereales par la 

 Ann. .scienc. nat. Bet. VITT. ser. VoLs. XIV. and XV. Paris 1902. 

 •esb. d. Sonderausschussos f. Pflanzen.schutz. Deutsche Landw. Ges. 1905 



