78o ECOLOGY 



nigrum) in which a bud developed where the two calluses fused, growing 

 subsequently into a shoot that combined the characters of both stock 

 and scion; to the new form thus produced there was given the name 

 Solanum tubingense. Several such forms have developed, some of which 

 are nearly intermediate between the parent species, while others more 

 closely resemble either the nightshade or the tomato. Seedlings revert 

 to one or the other of the parent forms. Shoots sometimes arise in 

 which a part is like the nightshade and a part like the tomato; such 

 forms have been termed chimeras. 



Much difference of opinion has arisen concerning the interpretation of these 

 striking results. One theory is that the new productions (such as Solanum tubin- 

 gense), which are not obvious chimeras, none the less resemble the latter in main- 

 taining the individuality of the two components, the portions of each being aggre- 

 gated into a sort of patchwork or mosaic; the new forms from this viewpoint are 

 regarded as hyperchimeras. Another theory has resulted from a study of forms 

 of Pelargonium with white margined leaves, in which it has been found that one 

 of the graft symbionts may serve as a sort of mantle for the other; the body of the 

 new form is composed entirely of one variety, while the epidermis alone, or the 

 epidermis with the hypodermis, is composed entirely of the other variety. These 

 remarkable forms have been termed periclinal chimeras. Already it has been 

 shown that Cytisus Adami is a periclinal chimera, the body being composed of C. 

 Laburnum and the epidermis of C. purpureus. There is reason to believe that 

 Crataegomespilus is to be explained similarly. In all of these cases, as in Solanum 

 tubingense, a seedling gives rise not to an intermediate form but to a form like one 

 of the parent symbionts. While most of the supposed graft hybrids thus appear 

 to be periclinal chimeras, there are some investigators who still hold to the reality of 

 graft hybrids; in the last analysis a graft hybrid should differ from all kinds of 

 chimeras in the merging of the protoplasm of the two graft symbionts. Such 

 merging has not as yet been demonstrated. 



Galls. — The influence of parasites upon their hosts. — When a para- 

 site attacks another organism, the activities of the latter may be ac- 

 celerated or diminished. For example, respiration and transpiration 

 commonly are increased and synthesis commonly is decreased. Para- 

 sites often secrete deleterious substances which, like many poisons, at 

 first excite various activities, while an increase of these substances causes 

 depression and even local or general death. In other cases the injury 

 caused by parasites consists chiefly in the removal of foods and food 

 materials from the host, which may in consequence be starved and 

 depauperate. 



Various characteristics of galls. — The most conspicuous influence of 

 parasites on hosts is in connection with gall formation. A gall is a struc- 



