708 BIOLOGICAL EFFECTS OF RADIATION 



long-day plants. However, certain southern forms of flax were markedly 

 retarded by a short day, and in Pisum certain southern forms suffered 

 more from a short day than did the northern forms. 



Kuznetzova (39) has made a study of the variation of the vegetation 

 period as affected by geographical location, based on extensive experi- 

 ments under the direction of Vavilov in which 185 winter and spring 

 varieties of various cultivated plants, mostly pure lines, were grown at 

 115 stations representing all parts of the Union of Socialistic Soviet 

 Repubhcs during the period 1923 to 1927. The complete developmental 

 period of the plant was divided into three phases: (a) from planting to 

 appearance of the young shoots; (6) from appearance of the shoots to 

 the flow^ering or earing stage; (c) from flowering or earing to ripening 

 of the seed. The duration of the first phase is dependent on various 

 environmental conditions, including those of a local, purely accidental 

 character, the chief factors being temperature and rainfall. As regards 

 duration of the second phase in relation to geographical location, all 

 plants tested were found to fall into three groups: (a) those in which the 

 time required for completion of this phase of development decreases with 

 increase in geographical latitude (long-day plants) ; (6) those in which the 

 duration of this phase decreases with decrease in latitude (short-day 

 plants) ; (c) those which are only slightly affected by geographical factors 

 (indeterminate or neutral plants). At the same latitude and with 

 simultaneous planting the duration of the second phase is regulated 

 chiefly by temperature. With equal conditions of temperature the dura- 

 tion of the second phase of development in different latitudes and in the 

 same latitude but at different altitudes is determined by the number of 

 hours of sunlight during the day. Other factors such as humidity, 

 cloudiness, and soil fertilization play only an insignificant role in such 

 principal phases of development as earing or flowering or they are of an 

 accidental character. Within the species there is distinct relationship 

 between the origin of each plant form and the degree to which it responds 

 to geographical factors. With respect to the third phase of development 

 the behavior of all plants is the same, the rate of development increasing 

 with decrease in latitude and, likewise, with decrease in altitude. The 

 degree of response, however, varies with the variety. The chief factors 

 influencing geographical variation in the length of the third phase of 

 development are temperature, rainfall, and relative humidity. In 

 contrast with its outstanding importance in the second phase, light has 

 no perceptible influence on the third phase of development. It is to be 

 noted, however, that this latter conclusion is not entirely in accord with 

 results reported by Garner and Allard and others. 



To ascertain whether the length-of-day factor might be responsible 

 for the fact that Australian varieties of wheat are early and do not succeed 

 well in England and, similarly, that English varieties are late and give 



