METHODS OF EXPERIMENTATION WITH COCKLEBUR 73 



experimental results. Flowering response of cocklcburs is fairly 

 sensitive to light intensity, and they respond best under full sunlight, 

 so it has been our practice not to apply shading to the greenhouse 

 glass during summer. The higher temperatures seem preferable to 

 low light intensities. 



Detailed studies on the effects of temperature and certain other 

 factors require better environmental control than that found in most 

 greenhouses. Thus controlled environment growth chambers or 

 rooms are coming into more frequent use. A number of such 

 installations constituting a single facility is referred to as a phytotron. 

 In any case the expense is very high, the light intensities are relatively 

 low (compared to sunlight), light quality is a serious problem, and 

 yet the results obtained with such facilities seem to make them well 

 worth the cost, as indicated by a number of experiments presented in 

 this book. Figure 5-2 shows one of our eight chambers at Colorado 

 State University. 



In some cases the physical facilities for studies on photoperiodism 

 deviate widely from those described here. The duckweed (Lemna 

 perpusilla, 6746), for example, is a short-day plant which has been 

 studied in some detail by W. Hillman (3) then at Yale University. 

 All of his studies are carried out in the laboratory with the small 

 plants floating on nutrient solutions. 



2. The Seeds 



Seeds of many species sensitive to day-length may be purchased 

 from local dealers. In North America, it is usually possible to collect 

 cocklebur seeds from weedy waste places in the fall of the year. The 

 seeds used in most studies in the United States were originally 

 collected near Chicago, Illinois, and they have since been propagated 

 by workers at the University of California at Los Angeles, the 

 California Institute of Technology in Pasadena, and other locations. 

 Cockleburs collected in more southern latitudes (around Los Angeles 

 for example) are sometimes not uniformly sensitive to photoperiod. 

 The days are not as long in summer, and apparently plants have not 

 been as carefully selected by nature. Various other plants sensitive 

 to photoperiod are also weeds and may be collected in nature. 



Seeds may be planted directly in the pots, perhaps two or three 

 to the pot, thinning to a single plant after germination. We germinate 

 the seeds in flats, subsequently transplanting them to pots, making 



