DORMANCY IN SEEDS 97 



grafting a portion of the normal stem on the dwarfs or a portion of the 

 dwarf stem on the normal seedlings had no effect whatsoever on the sub- 

 sequent growth of the normal seedling or in overcoming dwarfishness. 

 When the growing tip of a dwarfish seedling was grafted on a normal 

 seedling, the subsequent growth of this tip remained dwarfish. Thus the 

 seat of this dwarfishness is in the growing tip and not in the root or stem. 

 The breaking of seed dormancy in the peach by low temperature is in a 

 sense a treatment for overcoming bud dormancy, for normal development 

 is obtained by subjecting either the seed or the dwarfish seedling to the 

 required period at low temperature. 



Chemical and physiological changes occurring in dormant embryos during 

 low-temperature after-ripening. There have been a number of investiga- 

 tions of the chemical and physiological changes occurring in seeds with 

 dormant embryos during after- ripening at low temperatures. Eckerson ^^ 

 studied several species of Crataegus seeds using microscopic, chemical, and 

 physiological methods. The storage substances in these are in the form 

 of proteins and fats with little soluble sugar and no other assimilable car- 

 bohydrates. Also the foods are stored almost entirely in the cotyledons. 

 The initial change is increased acidity. Correlated with this is an increased 

 water-holding power; increased catalase and peroxidase activity occurred 

 as after-ripening progressed. Soaking the seeds in dilute acids hastened 

 the after-ripening and the changes mentioned above. Eckerson does not 

 know whether the increased acidity is a correlative change with after- 

 ripening or whether it holds a causal relation to after-ripening. There is 

 some evidence for the latter. There was also a great increase in water 

 absorption by the embryo as after-ripening progressed. 



Jones ^^ studied sugar-maple seeds which have proteins and fats as stor- 

 age material but also contain more than 6 per cent cane sugar. The best 

 after-ripening temperature is 5° C (41° F) and several weeks are required 

 to complete the process. During after-ripening there is a great increase 

 in catalase, a considerable increase in reducing sugar, and a slight increase 

 in peroxidase. There is no increase in water-absorbing power and the 

 embryo is alkaline in both mature and after-ripened seeds. Pack ^^' ^"' ^^ 

 studied the after-ripening of seeds of several species of Juniperus. These 

 seeds have their stored foods in the form of proteins and fats with little 

 sugar and no starch. The optimum temperature for after-ripening is 5° C 

 (41° F) and the time required is about 100 days. Recently Webster and 

 Ratliffe ^^^ have shortened the stratification period of Juniperus virginiana 

 seeds to less than two months by treating the seeds with a weak solution 

 of lye before stratifying. Pack ^^' p-^^ lists the following changes that 

 occur in the seeds of Juniperus during low-temperature after-ripening: 

 "(1) rather rapid and complete imbibition, followed by a steady slow 

 decrease in water content during after-ripening or until near germination; 

 (2) increased H+ ion concentration, especially of the embryo; (3) an incre- 

 ment of titratable acid; (4) a steady and enormous increase in the degree 



