I 10 GEORGE O. SHINJI. 



VIII. GENERAL DISCUSSION. 



As the result of our experiments, we have been able to dis- 

 tinguish two groups of substances, one that produces and the 

 other that does not produce winged aphids. Clarke (1901), 

 Neiils (1912) and Woodwortli (1908) held that the presence of 

 magnesium in the plant tissue might be responsible for the 

 phenomenon under discussion. My experiments have shown 

 that the magnesium salts are not the sole substances that produce 

 the winged aphids. Other substances, namely, the salts of 

 heavy metals, had a similar effect on the development of wings. 

 Consequently, it becomes clear that what is really responsible 

 for the appearance of the winged aphids is the excess of magne- 

 sium and other wing-producing substances over the non-wing- 

 producing substances contained in the plant sap. 



Morgan (1909) presented an analysis of the leaves of plants, 

 the rose and the maple, at two different seasons of the year, 

 namely, June and October. His table showed, as we should ex- 

 pect, a marked difference in the amount of ash contained in the 

 leaves of the two kinds of plants at the same time of the year. A 

 similar difference was also pointed out in the case of the same 

 plant at two different seasons of the year. The chemical analy- 

 sis of plants is, then, in accordance with the general observation 

 in the field that the alate forms are more abundant during the 

 autumn than in the spring and that the winged forms of the same 

 species are more abundant on one kind of plant than on another 

 even at the same period of the year. 



Morgan's table is also valuable in estimating the proximate 

 amount of wing-developing substances that may be effective. 

 According to the analysis, the percentages of ash contained in the 

 fresh rose leaves in October and June are 3.61 and 3.69, respec- 

 tively. The difference between these two figures, or 0.541 

 per cent., may then be regarded as the maximum amount of the 

 wing-developing substances which would be effective when added 

 to the spring leaves. 



I have already given an explanation of why the drying of 

 plants produced alate forms as reported by Woodworth (1908). 

 However, I did not discuss the secondary effect of temperature 

 on the development of the wing. Recently Ewing (1916) stated 



