84 Minnesota Plant Diseases. 



excites them to growth, so that where normally only one 

 kind of floral leaf is developed, two may be produced in the 

 diseased flower. The stamens may be produced in pistillate 

 flowers or pistils produced in staminate flowers. In still an- 

 other case stamens may by the influence of a fungus parasite 

 be transformed into petal-like bodies, thus producing a doub- 

 ling of petals. 



(d) All of the eflfects of parasitism mentioned thus far have 

 been either changes in normally developed organs or the 

 growth of undeveloped beginnings of parts. There are known 

 at present only a very few cases where a fungus excites the 

 formation of absolutely new organs. In these cases the organs 

 are produced by the host only when the fungus is present and 

 they have to do solely with aiding the parasite in the produc- 

 tion and dissemination of its spores. Such new organs are 

 known on a cone-bearing tree of Japan closely related to our 

 own white cedar and the fungus causing the formation of new 

 organs is a rust. 



Effects of parasitic fungi on tissues and structure of hosts. 

 It has already been said that fungi may cause increase in size of 

 plant parts. This increase in size is effected in two ways : first, 

 by an abnormal multiplication of the cells of the tissues affected 

 which takes place under stimulus from the fungus, and second, 

 by the enlargement of each cell. Both processes may go on at 

 the same time. 



Fungi act differently in their invasion of tissues and each 

 has its own method of attack. This is noticeable in the effect 

 upon the leaf-green of plants. Some fungi cause a decrease in 

 the amount of leaf-green found in the host plant, often effecting 

 its complete disappearance. Such plant-parts have a yellowish 

 color. Certain rusts have such an effect upon their hosts. 

 One may find other cases where only a partial decrease of leaf- 

 green occurs as in the balsam-fir needles on the witches'-broom, 

 also produced by a rust fungus. On the other hand the fungus 

 may excite the tissues to the production of an extraordinary 

 amount of leaf-green, or to the retention of leaf-green after sur- 

 rounding parts have lost it. Such green spots on leaves have 

 been known as green islands and are striking examples of the 

 unification of fungus and host-parts into a virtual individual. 



