366 Morpho genetic Factors 



for example, Scully, Parker, and Borthwick ( 1945 ) observed that, 

 with plants grown at photoperiods longer than necessary for bulb pro- 

 duction, variations in nitrogen had little effect, but when the photoperiod 

 was close to the critical one for bulbs, bulb development was consider- 

 ably greater at low nitrogen levels than at high ones. 



Cohen ( 1953 ) reports the effect of a nitrogen compound, ammonia, on 

 the development of the slime molds Dictyostelium and Polysphondylium, 

 two members of the Acrasiaceae ( p. 223 ) . Treatment with a low concen- 

 tration of ammonia greatly simplifies the morphogenetic expression of 

 these forms and reduces the degree of their differentiation. The sorocarps 

 produced resemble those of the genera Guttulina and Guttulinopsis. 



Carbohydrate-Nitrogen Ratio. Many other instances of the develop- 

 mental effects of nitrogen could be cited. More widespread than these 

 changes produced by nitrogen directly, however, are the ones that re- 

 sult from the balance between nitrogen and carbohydrates. Horticul- 

 turists know that nitrogen stimulates vegetative growth, that weakly 

 vegetative plants tend to flower early, and that in vigorously vegetative 

 ones flowering is either scanty or does not occur until the amount of 

 available nitrogen is reduced. This problem was first studied intensivelv 

 by Kraus and Kraybill (1918) in tomatoes. They found that in strongly 

 vegetative plants bearing few flowers or fruit the C/N ratio was low. 

 Those producing fruit abundantly had a high C/N ratio but had been 

 given a good supply of nitrogen at the beginning of their growth. They 

 interpret these facts to mean that when nitrogen is abundant all the 

 carbohydrate produced by the plant will be used in forming new vege- 

 tative tissue. If nitrogen is in short supply, however, carbohydrates will 

 tend to accumulate, and when the C/N ratio becomes high enough, the 

 development of reproductive structures will be stimulated. This hap- 

 pens early in weakly vegetative individuals, but such plants are too small 

 to produce a large crop of fruit. Maximum yield results from an abundant 

 supply of nitrogen at first and an active production of carbohydrates 

 later by the fully grown vegetative structures. This is what normally 

 happens under favorable conditions. The important morphogenetic as- 

 pect of this hypothesis is that it maintains that the differentiation of 

 reproductive organs is dependent on the accumulation of carbohydrate in 

 the plant. Until this happens, vegetative structures take priority over 

 reproductive ones in the use of available carbohydrate. 



In general, the younger the tissue, the lower is the C/N ratio. From 

 a low point in the seedling it increases as foliage develops and as the 

 ratio of top to root grows larger until it reaches the point where reproduc- 

 tive structures are formed. This would explain why flowering normally 

 is deferred until the plant has reached a considerable vegetative develop- 

 ment. 



