CHAPTER 5 



METHODS OF EXPERIMENTATION 

 WITH COCKLEBUR 



Having completed the survey of some principles of photoperiodism 

 as they apply to plants in general, we are now ready to begin our 

 discussion of the mechanisms involved in the initiation of flowers, 

 concentrating somewhat on a single species: the cocklebur. The 

 usual procedure is to cause flowering of this short-day plant by 

 exposing it to an uninterrupted dark period, carrying out various 

 treatments in conjunction with this, and then studying the subsequent 

 extent of flowering in treated and control plants. 



There are many variations upon this theme; many treatments which 

 may be given, and many details which must be considered. The 

 primary purpose of this chapter is to provide insight into this 

 experimental approach. I will describe the facilities and procedures 

 which we use in our laboratory at Colorado State University, along 

 with some of the methods used by other workers in attacking prob- 

 lems which we have not studied in Colorado. While our methods are 

 special in certain ways, they are representative of the basic approach 

 to photoperiodism, and as such they might serve as a framework for 

 evaluation of the experiments described in the rest of the book. 



In past years I have received a number of letters from high-school 

 students asking how they might carry out experiments in photo- 

 periodism. The following chapter is not written as a cook-book for 

 such undertakings, but a little bit of the ingenuity displayed by such 

 students should easily adapt the principles to amateur study. 



I. Growing the Plants 



1. Facilities for Growing Plants 



Figure 5-1 is an interior view of our greenhouse at Colorado 

 State University. Temperature control by evaporative cooling (as in 

 the figure) or air conditioning might be expected to improve the 



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