784 



ECOLOGY 



trees (as the white birch, the hackberry, and various conifers) by Exoascus and by 

 other fungi, and by the dwarf mistletoe, Arceuthobiutn pusillum. In these galls 

 many small twigs diverge from the part infected, thus manifesting a resemblance 

 to a broom or brush. In the case of Exoascus the mycelium hibernates, so that 

 the fungus recurs season after season. Other important fungus galls are : the black 

 knot of the cherry (caused by Plcrwrightia, fig. iioo); the ergot of rye and other 



y^ 



1100 



Figs. 1097-1100. — 1097-1099 gall formation in a goldenrod (Solidago serotina): 

 1097, the apical portion of a plant that has been attacked by an insect (Cccidomyia 

 Solidaginis); such galls check stem elongation and prevent flowering; note the variation 

 in leaf form; 1098, an ordinary leaf; 1099, a gall leaf or leaf-complex made up of a num- 

 ber of coalesced leaves ; IIOO, a "black knot" on a branch of the choke cherry {Prunus 

 virginiana)^ an example of gall formation through fungal influence, the stimulating 

 fungus being Plowrighlia morbosa ; the swollen black mass is known as a stroma, and it 

 contains many fructifications known as perithecia. 



grasses (caused by Claviceps), in which black protruding bodies, the sclerotia, 

 replace the grains ; the cedar apple of Juniperus virginiana (caused by Cymnospo- 

 rangium); and the leaf and flower galls of the Ericaceae (caused by Exobasidium). 

 Fungi also occasion root galls in the cabbage and in the alder, and bacteria occasion 

 galls on the roots of leguminous plants (p. 787). Sometimes fungi cause pro- 

 nounced changes in plant habit. For example, the prostrate herbs, Euphorbia 

 maculata and E. polygonifotia, become erect when infested with a certain rust ; 



