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THE AMERICAN NATURALIST [Vol. XLII 



arvense, P. cladoplithiriis as the cause of gray-hirsute mal- 

 formations of the shoots of Solarium Dulcamara, and P. 

 genista as the cause of malformations of the tips of the 

 shoots and abnormal hirsuteness of the buds of Genista 

 pilosa and Sarothamnus scoparius. The virescence of 

 the inflorescence of different species of Arabis, due to 

 Aphides, was studied by Peyritsch. 45 De Vries 46 ascribes 

 an epidemic of virescence among the plants in his experi- 

 mental garden to an original infection caused by Phytop- 

 tus, though he was unable to demonstrate the presence of 

 the mite. Finally Molliard 47 investigated the influence of 

 fungi and insects causing floral cecidia upon the repro- 

 ductive cells. 



From the above it will be gathered that there exists a 

 very definite relation between malformation in plants and 

 gall-insects. For this reason a large number of strikingly 

 abnormal plants of the horse-weed, Erigeron canadensis, 

 growing within a narrowly circumscribed area in the im- 

 mediate vicinity of St. Louis, Mo., attracted immediate 

 attention and awakened considerable interest. Being 

 found in January, nothing but the dried parts remained, 

 which made observation easier. All of the plants were 

 abnormal. Among them two distinct types could be dis- 

 tinguished. These types agreed in one particular. From 

 the ground to a place 2|-3 feet above the soil, the plants 

 were normal. It was above this point that the abnor- 

 mality presented itself. In the first type, when the plant 

 had reached a height of from 2^-3 feet above the ground 

 it had evidently experienced a check. The main stem 

 terminated here in a very much dried-up shoot but an 

 inch or less in length, showing that it never had an op- 

 portunity to perfect its woody tissue. Just below this 

 point numerous small side shoots occurred. These side 



"Peyritsch, J. Zur ^tiologie der Chloranthien einiger Arabis Arten. 



