270 THE SMALL GRAINS 



matic factors are usually considered, the phenology of the 

 cereals will be discussed in this place, although varying 

 soil conditions would have some part, probably, in a full 

 treatment of the subject. 



As before stated (14), there are four important stages 

 in the life of the plant; the seedling, jointing, heading 

 or flowering, and ripening stages. The jointing stage is 

 so extended and so irregular as to individual plants in the 

 same field, that it is practically impossible to fix an aver- 

 age date for it, and therefore observations of this stage 

 are not usually recorded. The actual dates of occur- 

 rences of these phenomena are only important as means 

 of measuring the periods between. The period from the 

 date of emergence of the seedling above ground (the first 

 visible sign of germination) to the date of heading will be 

 termed here the vegetative period. The period from date 

 of heading (equivalent to date of flowering) to date of 

 ripening is already known as the fruiting period, while 

 the sum of these two is the entire period of growth. 1 

 The varying lengths of these periods or their ratios with 

 each other are found to be correlated with certain char- 

 acteristics of the cereal plants in an interesting way. 



286. Variations in the period of growth. While the 

 life period of spring grains decreases northward, the 

 period from date of emergence to date of ripening of 

 winter grains decreases southward. The explanation 

 usually given for the first condition is the greater length 

 of summer days toward the north, thus permitting more 

 sunlight. This fact is at least partially counterbalanced 

 by the corresponding one of less intensity of the sunlight 

 in the north, because of slanting of the sun's rays. Prob- 



1 In winter crops there is also properly a resting period 



(see 287-288). 



