Plants & Human Affairs - Introduction
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Pod Size in Rapid Cycling Brassica rapa (RCBr)

 Learning Objectives:  Upon completion of this lab you should be able to:

  1. grow an RCBr ("Fast Plant") from seed to seed
  2. describe the life history of Brassica rapa using terms such as radicle, hypocotyl, cotyledon, epicotyl, node, internode, pedicel, receptacle, sepals, petal, androecium (or stamen); filament, anther, gynoecium (or pistil), stigma, style, ovary, nectary, pollination, silique (pod), raceme, inflorescence, fertilization
  3. cross pollinate a flower
  4. describe the effect of plant hormones on RCBr fruit expansion
  5. prepare computerized bar graphs and scatter plots
  6. perform simple statistical tests (mean, standard deviation, correlation)

    The purpose of this lab is two-fold.  First, this lab will provide the opportunity for you to study the growth of a plant throughout its entire life cycle.  To learn more about the history and biology of RCBr visit the visit the Wisconsin Fast Plants web site.  This site provides a wealth of information, including the on-line version of Dr. Williams article and slides/images of the plants.  The second major purpose of this lab is to use Fast Plants as a model experimental organism to study the physiological mechanisms involved in fruit growth.  

Exercise 1:   Structure (Morphology) of Rapid-Cycling Brassica rapa Pods
Recall that the pod is a fruit which is derived from the pistil of the flower.  As a result, in many flowers we can still observed the remnants of the pistil parts.  The purpose of this exercise is to familiarize you with the structure of RCBr pods and to relate the pod structure to that of the pistil.


  1. On a separate sheet of paper, sketch the external features of the pod (label stigma, style, ovary, receptacle).  At the bottom of the page, label this diagram "Fig. 1: External morphology of RCBr pods."
  2. Measure the length of the pod in millimeters ________ (What, exactly, will you measure?) and record your data in Table 1.
  1. To study the internal anatomy we will carefully dissect open the pod.  Gently pry off one of the halves (carpels). Be sure to collect any seeds that fall out of the pod on the sticky tape provided.  Save the seeds.  On the back of your paper that contains Fig 1., sketch the internal anatomy of the pod and the arrangement of seeds in the pod.  Label this diagram "Fig 2: Internal morphology of RCBr pods."  Locate and label on the diagram the funiculus (stalk that attaches seed to pod), septum (partition between fruit halves), placenta (region where the funiculus attaches the seed to the fruit wall).
  2. Count the seeds that were in the pod and record in Table 1. Be sure to SAVE the seeds. 
  3. Complete Table 1 by collecting data from other members in the class.

Table 1: Size and seed number of wild type RCBr pods

Pod length (mm) Seed Number   Pod Length (mm) Seed Number


Exercise 2.  RCBr Pod Size Analysis
 If you scan Table 1 it should be obvious that some pods are long while others are short.  Or in other words, pod length exhibits considerable variation.  After collecting such data, a scientist will typically use descriptive statistics to help summarize and depict the central tendencies in these data.  A scientist will want to know the answers to such questions as: How big is the average pod?   What is the largest pod?  What is the smallest pod?  How much variation exists in pod size? 


  1. Complete Table 2. A spreadsheet program such as Excel can simplify these calculations.

  2. Prepare a histogram of pod length.  Label it "Figure 1 - Frequency Distribution of RCBr pods."  To do so, first sort your data in size classes in Table 3.  Then plot a histogram of these data.

Data & Analysis:

Table 2.  RCBr Pod Statistics
Maximum pod length (mm)  
minimum pod length (mm)  
average pod length (mm)  
standard deviation  


Table 3.  Pod Frequency Distribution
Size Category (mm) Number of individuals (observations)
0 - 5  
5 - 10  
11 - 15  
16 - 20  
21 - 25  
26 - 30  
31 - 35  
36 - 40  
41- 45  
46 - 50  

Exercise 3: 
What determines pod size in RCBr? 
      Why are some pods longer than others? 

HypothesesMake a list of several potential hypotheses to explain pod length variation in RCBr.






    Consider the hypothesis - "Pod size is related to seed number.  Longer pods have more seeds."  We can test this hypothesis with the data we previously collected (Table 1).  We will analyze these data graphical.  Make a graph plotting both variables.  Label this graph Figure 2 and give it a proper caption. 

    To determine the relationship between the two variables we can draw the best fit line (regression) and do a statistical test called a correlation.  In class I will show you how to use Excel to perform these tests.  Record the correlation coefficient (r2).


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Last updated:  07/30/2005 / � Copyright  by SG Saupe