METHODS OF EXPERIMENTATION WITH COCKLEBUR 77 



and the rate of development of the flowers is increased. The flowering 

 response increases right up to the point where the plant begins to be 

 damaged by too much fertilizer. Other studies indicate that phos- 

 phorus and nitrogen are especially important in improving the 

 flowering response of cocklebur (25), but as mentioned in Chapter 4, 

 some species flower best when they are Hmited in the amount of 

 fertilizer that they receive. 



We have found carts, such as that shown in Fig. 5-1, to be handy 

 for most greenhouse procedures as well as for the experiments 

 themselves. Our carts have four freely pivoting casters, which make 

 them easy to move in any direction. Carts are commercially available, 

 but we obtained the desired design only by having the carts built 

 locally. 



II. Preparing Plants for Experimentation 



1. Selection for Uniformity 



Plants must have reached an age of proper sensitivity. This seems 

 to depend considerably upon growing conditions, but 60-day-old 

 plants are almost always quite sensitive, and under optimum condi- 

 tions, even 30-day-old plants will respond rather nicely. After plants 

 are chosen for their general condition, age, height, etc., we sort them 

 further according to leaf size. At an early stage of experimentation, 

 I measured the length of the leaf midribs from the base to the tip of 

 the leaf on 100 plants. The smallest leaf which was longer than 1 cm 

 was called leaf No. 1, the next largest leaf, leaf No. 2, and so on. 

 The average leaf lengths are shown in Fig. 5-4. Plants were then 

 chosen on which all the No. 3 leaves were between 6.9 and 8.5 cm. 

 In one representative group of these plants all of the leaves except 

 No. 1 were trimmed off", in another all leaves except No. 2 were 

 removed, and so on. Plants trimmed in this way were then given an 

 inductive dark period, and their flowering was checked a few days 

 later. The results are also shown in Fig. 5-4. The No. 3 leaf was 

 clearly the most sensitive to photoperiodic induction. 



It is often handy to use plants which have been trimmed so that 

 only one leaf remains. This makes application of chemicals simpler, 

 and it is especially helpful in light studies where intensity at the leaf 

 level must be determined. We use plants trimmed in this way as a 

 standard procedure. Since the No. 3 leaf is most sensitive, it is used 



