1912] Morphology and Biology of Insect Galls 297 



A CONTRIBUTION TO THE MORPHOLOGY AND BIOLOGY 



OF INSECT GALLS. 



By A. CosENS, M.A. 



The earliest explanations offered to account for the origin of galls 

 are more or less fanciful, as must needs be, since nothing was known con- 

 cerning the reciprocal relations of host and parasite involved in their 

 life histories. Some of these theories dating from a less scientific age 

 are crystallized in the popular names attached to certain classes of these 

 structures. Thus the bristling masses of twigs, produced on several 

 species of trees by the stimulation of plant or animal parasites, are known 

 by some as "thunder bushes," a term that implies the expenditure of 

 electrical energy in their origin. Also the more common name, "witches' 

 brooms," indicates an equally imaginative explanation. 



With the gradual advances of science much of the mystery surround- 

 ing galls has been dispelled, but they still present many interesting 

 problems. Among these are two standing out so prominently that they 

 seem to include all others. The first of these concerns the causes that 

 are operative in producing the gall. It is now generally recognized that 

 its origin is directly ascribable to the stimulation of a parasi e, but the 

 problem still remains concerning the nature of the stimulus and the prin- 

 ciple governing the response. Concerning the nature of the stimulus, 

 Fockeu and his school ascribe it to mechanical means, while Kiister, 

 Kiistenmacher and others believe it to be referable to a chemical action. 

 With regard to the response by the host, the conventional view endows 

 the stimulated protoplasm with power to originate something foreign 

 and without a prototype in the normal host. But a view is developed 

 in this paper proposing an entirely different explanation, namely, that the 

 supposedly new types of organs, tissues, etc., are due to the awakening 

 of dormant characteristics in the protoplasm. 



The second problem deals with the apparent philanthropy that char- 

 acterizes the host plant in its care for the parasite. Concerning this, 

 several explanations have been offered. The parasite may be simply 

 taking advantage of structures thrown out by the plant in its own de- 

 fence, or as Adler^ figuratively expresses it, the besieger is making use 

 of the water in the moat in pushing forward the attack on the forti- 

 fications. Perchance, even what Darwin regarded as impossible has 

 taken place, and the plant is producing the gall entirely for the welfare of 

 the parasite. In opposition to this the theory is proposed by some 



