NUTRITION AND GROWTH 155 



reveal the "Ideologically interesting fact . . . that 

 those proteins, like casein, lactalbumin, and egg vitellin, 

 which are in nature concerned with the growth of 

 animals, all show a relatively high content of lysine." 

 The experience of these investigators with zein, 

 which lacks glycocoll, tryptophane and lysine, has 

 brought to light the fact that tryptophane is undoubt- 

 edly essential for maintenance and emphasizes anew 

 the significance of lysine as a growth promoting sub- 

 stance. One may also assume that a little lysine is 

 necessary for maintenance and this is ordinarily sup- 

 plied in sufficient amount by the traces in gliadin or 

 (in the zein and tryptophane experiments) by traces 

 in protein-free milk protein or from the tissues them- 

 selves. In an earlier portion of this chapter were 

 pointed out in some detail the experiments of Will- 

 cock and Hopkins with zein, with and without addi- 

 tion of tryptophane. They found that zein as the 

 only protein in the dietary cannot maintain growth in 

 the young animal nor even support life. The addi- 

 tion of tryptophane resulted in prolonging life with- 

 out causing a resumption of the growth impulse. 

 The outcome of the work of Osborne and Mendel 

 with zein alone is best shown in the chart, Fig. 11. 

 The large number of experiments shown here yielded 

 concordant results and show that neither maintenance 

 nor growth can be secured when zein is the only pro- 

 tein ingested. When tryptophane is added to the zein 

 food mixture, maintenance of body weight follows, as 

 may be seen from Fig. 12. Addition of both trypto- 



