60 MASSACHUSETTS HORTICULTURAL SOCIETY. 



is considerably affected. The latter is perhaps brought about 

 commonly through the reduced sugar content, whereby the acidity 

 becomes more apparent. In the case of the rust of certain brome 

 grasses where etiolated plants are not readily infected, Professor 

 Marshall Ward, took the view that the fungus is closely associated 

 with the nutrition of the plant and requires a vigorous nutrition. 

 Such vigorous "nutrition" could not be p^o^'ided when etiolation 

 effects were marked. Therefore, infection fails under such cir- 

 cumstances. It is true that from many points of view etiolated or 

 shaded plants may be regarded as "ill adapted to withstand the 

 ordinary exigencies of plant life," and nevertheless, it must be 

 remembered that during bright sunlight there is an unbalanced con- 

 dition of the environment. The light is far more intense than is 

 necessary in order that the plant may use all of the carbon dioxide 

 which comes in contact with its leaf surface. The additional energy 

 of this unnecessary sunlight produces beyond all doubt certain 

 destructive effects, and these may themselves predispose to diseases 

 of certain t^'jDcs. Infection by means of uredospores, aecidiospores 

 and various other forms of spores takes place through the stomates. 

 These stomates are apparently important in the regulation of the 

 amount of moisture given off by the plant through evaporation, but 

 they are most important in the interchange of gases between the 

 outside air and that contained within the plant. In consequence 

 they are open during the sunlight, and it is safe to. assume that in 

 many cases healthy tissues are more readily infected during sunlight 

 than at any other time. INIoreover, the growth of a h}T)ha seems 

 to be directed toward a source of moisture, so that if we imagine a 

 rust spore which has germinated in the dew condensed upon the 

 surface of the leaf during the night it will be directed as this dew 

 dries up in the early morning toward the open stoma, and entrance 

 will be effected. There is a difference, however, between the 

 entrance of the fungus and its growth within the tissues, in other 

 words, a difference between penetration and effective infection. 

 The conditions outside may alone practically determine the germi- 

 nation of the s])ore and its entrance to the stomatal caAity. The 

 chief relation, however, in the case of such plants as rusts and 

 other equally parasitic forms with the host is subsequent to pene- 

 tration. The infection tube may be stopped by various starvation 



