METHODS OF EXPERIMENTATION WITH ( OCKLEBUR 89 



3. Our Approach 



We are primarily interested in the act of induction, and thus would 

 like to know the amount of flowering hormone produced, but our 

 only measure of this amount is the appearance of the flower. 



The rate of development of the cocklebur flower (specifically the 

 male flower at the stem apex) seems to be primarily a function of the 

 amount of flowering hormone which arrives at the tip. This conclu- 

 sion, although a reasonable one, is probably based on a form of 

 circular logic. We will discuss it further along with some examples 

 in Chapter 9, but the basic idea is this : treatments which might be 

 expected to influence the amount of flowering hormone reaching the 

 tip also influence the subsequent rate of development of the bud. 



We need a numerical measurement for this rate of development. 

 This can be obtained by simply measuring the size of the buds at 

 intervals after induction, and such a procedure has often been used. 

 A better procedure is to establish a series of arbitrary stages of 

 development of the bud. As a graduate student, I made about 80 

 photomicrographs of developing buds and then arranged the prints 

 according to a logical sequence. The result was the series of 8 stages 

 of floral development which are illustrated in Fig. 5-8 and defined 

 in Table 5-4. The stages involve morphogenetic changes, and these 

 seem to be less influenced by the plant's general health than is the 



Table 5^. Criteria for the Determination of Flowering Stages 



IN THE Apical Bud (to become staminate) of 



Xanthium pennsylvanicum, Cocklebur 



Floral stage Criterion 



Vegetative. Shoot apex relatively flat and small. 



1 First clearly visible swelling of the shoot apex. 



2 Floral apex at least as high as broad, but not yet con- 

 stricted at the base. 



3 Floral apex constricted at the base, but no flower pri- 

 mordia yet visible. 



4 First visible flower primordia, covering up to the lower 

 one-quarter of the floral apex. 



5 Flower primordia covering from one to three-quarters 

 of the floral apex. 



6 Flower primordia covering all but the upper tip of the 

 floral apex. 



7 Floral apex completely covered by flower primordia. 

 Slightly to moderately pubescent. 



8 Very pubescent and showing some diff'erentiation of 

 flower parts. At least one millimeter basal diameter. 



