ANATOMICAL AND HISTOLOGICAL CHANGES 



IN RELATION TO 



VERNALIZATION AND PHOTOPERIODISM 



by 

 R. H. Roberts and B. Esther Struckmeyer 



University of IVisconiin 



The cause of flowering in plants has been the subject of a rapidly increasing 

 amount of literature in the past few decades. The majority of such papers have dealt 

 with the environmental factors employed to induce blossoming rather than with any 

 possible basic physiology of sexual reproduction. That is, such items of external 

 environment as the degrees of temperature and quality or duration of the daily light 

 have been more often the subject for investigation and emphasis than have the condi- 

 tions which might consistently occur within the plant when blossom primordia are 

 initiated. 



The opposite effects which comparable applications of nitrogen have upon the 

 blossoming and fruiting of apple trees which are in widely different stages of vigor 

 directed attention toward the possibility of using histological conditions and anatomi- 

 cal characters as an index of the beginning of reproductive processes (16, 21). 

 Observation of the photoperiodic efifects and particularly the influence of temperature 

 upon photoperiodism (27, 28, 30) increased interest in the question : Are there common 

 physiological conditions present within plants and also common physiological reactions 

 by different species of plants whenever sexual reproduction is initiated, even though 

 uses of different temperature, light, water or nitrogen nutrient conditions are needed 

 to bring about flowering? 



Four different lines of evidence have been secured which indicate a common 

 sequence of physiological events when blossom induction occurs: (1) The rhythm 

 of CO2 exchange was found to be unlike in flowering and non-flowering plants (24, 

 25). This was true of both long- and short-day plants. (2) The daily growth rate 

 and diurnal cycle of growth change very quickly when plants are placed in condi- 

 tions leading to blossom induction (11). (i) Root extension is greatly reduced when 

 plants enter the reproductive state. That is, top-root ratios of a number of plants 

 were found to be related to flowering of the plants and not to the day length used 

 to induce blossoming (32). (4) A large number of varieties of plants, including 

 day-neutral and long- and short-day types, and those with high or low temperature 

 preference exhibited common anatomical characters when placed in an environment 

 inducive to blossoming (32, 33, 36). A review and extension of these latter observa- 

 tions, together with a consideration of the available references to similar work and 

 their significance to photoperiod and vernalization is the subject of this paper. A 

 fifth physiological condition has recently been investigated (23). This is pigment 

 content. It has been found that the fractions of chlorophyll, carotene, violaxanthin 

 and xanthophyll present during the daytime as measured by the chromatographic 

 method do not appear to be related to blossoming. Also, pigment conditions during 

 the dark period are not consistently correlated to reproduction. It appears that the 

 diurnal cycle of pigment content, especially carotene, varies with day length rather 

 than with reproductive state. 



Experimental Evidence : — Interest in the possibility of the anatomical 

 structure of plant stems being associated with flowering arose from the 



