MOVEMENT AND REORGANIZATION OF CELLS. 25 



ripened parent stem. That these stem bulbs are the pro- 

 duct of reorganized cell-matter, which, under other con- 

 ditions, would have spread out into long, thin, aerial, and 

 true leaves, is quite evident from the fact that tney are 

 only produced at the point where an embryo leaf-bud 

 had formed a junction with the stem. It is not only 

 in the propagation of plants under artificial conditions 

 that we find great uniform- 

 ity in the movement of cell- 

 matter proceeding from 

 uniform causes, but it is 

 also observable in the re- 

 sults of attacks and injuries 

 inflicted by insects. In the 

 growth of what are called 



53. 



galTs'on plants, produced, 



so far as known, by the irri- 



tation caused in depositing 



eggs, or the presence of the 



larvae hatched therefrom, 



the results are so uniform 



that the entomologist is 



enabled, at a glance, to Fig. 



identify the inhabitants of galls by the structure and 



outward appearance of their dwellings. 



Why the irritation caused by the depositing of a few 

 minute eggs by a small four- winged black fly (Gynips 

 spongifica), on the leaf-stalk of the Black Qal^ should 

 cause cell-matter to rush to that part, and form a large 

 puffy gall an inch or more in diameter, and of a speci- 

 fic structure, differing widely from those produced by a 

 closely-related insect upon other species of the Oak, we 

 do not know ; but the fact that every distinct species of 

 Cynips produces a different gall is well known to every 

 entomologist. (Figure 7 a, larva in center; b, hole 

 where the fly escaped.) 



GALL. 



