258 PLANT GROWTH SUBSTANCES 



on the control medium did not continue their embryonic growth, but 

 germinated in about three or four days, forming seedhngs which are 

 abnormally small and spindling. This type of growth is indicated when 

 the shoot height is large and the percentage of dry matter is small 

 (around 10 per cent). If i per cent of casein hydrolysate is added to the 

 medium, very young embryos fail to germinate and older ones are 

 retarded so that the shoot height is reduced relative to the control; the 

 wet and dry weights are also reduced, but the percentage of dry matter 

 is high. The effect of the amino acid mixture containing phosphate 

 and sodium chloride is indistinguishable from that of the casein hydrol- 

 ysate. Each component alone, however, fails to duplicate the effect 

 of the complete mixture. The added phosphate reduces the height 

 slightly, while the amino acids and sodium chloride are more effective 

 in this respect. Note that, in general, the shoot height is reduced on 

 media with high osmotic pressure. The percentages of dry matter of 

 embryos grown on media containing each of the three supplements 

 singly are intermediate between those of the control and the complete 

 mixture. 



A similar experiment, using the components in combinations of two, 

 showed that none of the three possible pairs reproduced the results 

 given by the casein hydrolysate, although the combination of amino 

 acids and sodium chloride with an osmotic pressure of about nine 

 atmospheres caused almost complete suppression of germination. Experi- 

 ments with high concentrations of sucrose or mannitol demonstrated 

 that the inhibition of germination could be attributed to the high osmotic 

 pressure of the medium, as shown in Table 2. On all three media of high 

 osmotic pressure the shoot height is low and the percentage of dry matter 

 is high. It may be concluded then that the germination-inhibiting effect 

 of casein hydrolysate is due to its osmotic pressure, for which the amino 

 acids and sodium chloride are mainly responsible. The rapid embryonic 

 growth which takes place on the casein hydrolysate medium is greater 

 than that occurring on the media supplemented with sucrose or mannitol; 

 the wet weight of the embryos on the casein hydrolysate medium is the 

 greatest, and the dry weights on the casein hydrolysate and sucrose 

 media, respectively, are greater than on the mannitol medium. Mannitol 

 is nutritionally inactive for barley embryos. It is thought, therefore, 

 that these weight differences result from the fact that casein hydrolysate 

 and, to a lesser extent, the additional sucrose serve as nutrients as well 



