314 



SCIENCE 



[Vol. LVI, No. 1446 



in yields of winter wheat. Apparently, from 

 their studies, the iniluences controlling the be- 

 ginning of the development which determined 

 final yielding ability of winter wheat occurred 

 in March, regardless of how favorable growing 

 conditions were for the rest of the season or 

 how severe conditions during the preceding 

 dormant period had been. Schafer and his asso- 

 ciates report (9) that Hybrid 128 will not head 

 out when planted later than March 11, and (10) 

 that Turkey Red will joint in October when 

 planted in April, while Hj'biid 128 will not. 

 McCall and Wanser (15) have found that 

 Jones Fife and similar wheats joint early in 

 the spring, while wheats of the Turkey Bed 

 type do not joint until a later date. 



These observations indicate that the winter 

 habit of wheat is caused by the absence of the 

 critical stimulus which is essential for the 

 initiation of the jointing stage. Though the 

 response to the stimulus may be affected and 

 altered by temperature and nutritional factors 

 and, under field conditions, apparently, has 

 been usually associated with these factoi-s, the 

 stimulus itself is, nevertheless, independent of 

 them and for any given locality is controlled 

 more by date than by current growing condi- 

 tions. The observations of Schafer and his 

 associates and of McCall and Wanser indicate 

 that the date of the occurrence of the stimulus 

 varies for different varieties, but for any given 

 variety is fairly constant in a given locality. 



In the light of the work of Garner and AUard 

 and of the results secured by the writer, aU of 

 this evidence indicates, in the case of winter 

 wheat, the stimulus for jointing to be a critical 

 photoperiod having a maximum limit. The 

 passing of this maximum limit results in a 

 spring-sown winter variety failing to joint 

 until the occurrence of shorter days during the 

 following fall or succeeding spring. The sea- 

 son at which jointing then takes place depends 

 on the occurrence of the length of day corre- 

 sponding to the necessary photoperiod and an 

 accompaniment of temperatures favorable for 

 growth. In any case, heading, which must be 

 preceded by jointing, is delayed until the fol- 

 lowing summer because of the longer day 

 necessary to start this stage of development. 



Although a preliminary experiment is always 

 restricted in scope, the close agreement between 



the results of this experiment and the analysis 

 of the supporting evidence makes possible a 

 few safe tentative conclusions. The develop- 

 ment of winter wheat requires a critical photo- 

 period for jointing and also a separate and 

 distinct critical photoperiod for heading. 

 Garner and Allard, working with dicotyledon- 

 ous plants, mention but one critical photo- 

 period. Although varying with species and 

 variety in the intensity of distinction, there 

 probably are for most monocotyledonous and 

 some dicotyledonous plants at least two critical 

 photoperiods, one for starting culm or stalk 

 development from the tillering or rosette stage 

 and another for starting the heading or bud- 

 ding and blossoming stage."- The photoperiods 

 for both responses probably have an optimum 

 with a maximum and a minimum limit, but 

 for winter wheat they are independent of each 

 other, do not overlap and vary with variety. 

 The northern limits of the distribution of 

 winter wheat are probably very largely con- 

 trolled by the relation of the date of the be- 

 ginning of the active growing season to the 

 date at which the longest day within the limits 

 of the critical photoperiod for jointing occurs 

 in that locality. 



While the photoperiods for jointing and 

 heading do not overlap and are entirely dis- 

 tinct in the case of winter wheat, they are not 

 so distinct in the case of spring wheats. In 

 the latter group the photoperiod for jointing 

 is of greater magnitude than in the former, 

 possibly without a maximum limit, and joint- 

 ing and heading are possible under more nearly 

 an identical photoperiodic stimulus. As a 

 result such varieties when sown in the spring 

 joint and head the same season. Photoperiod- 

 ism, therefore, is the key to the distinction be- 

 tween winter and spring wheats. 



Although no published evidence showing the 

 effects of photoperiodism in the development 

 of spring wheat is here referred to, there is, 

 nevertheless, an abundance of available mate- 

 rial, some of which will be nrentioned in a later 

 detailed report of experimental work now in 

 1 In an article published in Science (June 2, 

 1922) since the preparation of this paper Garner 

 and Allard recognize the two photoperiods for 

 dicotyledonous plants but do not mention or con- 

 sider monocotyledonous plants. 



