3. Two to four thicknesses of white or colorfast blotters, filter paper, or paper towels. 



4. Black sateen cloth bags made of two layers of cloth large enough to hold the dishes. An alternate method 

 is to wrap the dishes in two layers of aluminum foil. 



PROCEDURE: 



1. Collect seeds of several kinds of local weeds. In general, seeds will retain their viability fairly well when 

 stored dry in a refrigerator. Some suggested seeds known to be light-sensitive are peppergrass (Lepidium 

 virginicum and L. densiflorum), henbit (Lamium amplexicaule) , and hedge mustard (Sisymbrium offi- 

 cinale). Other weed seeds worthy of investigation are shepherds-purse (Capsella bursa-pastoris), yellow- 

 rocket (Barbarea vulgaris), tumble-mustard (Sisymbrium altissimum), chickweed (a species of Stellaria 

 or Cerastium), sheep sorrel (Rumex acetosella), the small-seeded cactuses, and many others. 



2. Four dishes should be used for each kind of seeds tested. 



3. Cut the blotters to fit the dishes and presoak overnight (about 16 hours) by putting enough tapwater 

 into the dishes to flood the blotters. 



4. After the blotters are thoroughly soaked, pour off excess water and evenly distribute 100 seeds over the 

 top blotter in each dish. 



5. Immediately after the seeds are distributed, cover the dishes with their lids and place dishes in the black 

 cloth bags. 



6. Allow the seeds to imbibe water in the dark for a period of 16 to 24 hours, then begin treatments. 



7. Treatments: 



(a) Dishes 1 and 2 should be kept at about 70° F. during the entire period of the demonstration. Dishes 

 3 and 4 should be held at about 60° during the 16- to 24-hour imbibition period, then transferred to a 

 temperature of 77° for the remainder of the demonstration. 



(b) Dishes 1 and 3 are placed in the black cloth bags at the time of planting and left there throughout 

 the demonstration. These will serve as "dark controls." 



(c) Dishes 2 and 4 should be placed in unfiltered sunlight for 1 hour each day. 



(d) When the seeds in dishes 2 and 4 have germinated, the other dishes are removed from the black cloth 

 bags and the number of germinated seeds are counted and recorded for each treatment. 



OBSERVATIONS: 



Record the number of days required for germination, the temperature, light conditions, and so forth. Count 

 the number of seeds that germinate under each treatment and record as percentage of germination. When a 

 light-sensitive seed is found, demonstrations can be designed to determine how much light the seeds require, 

 how many times they must be exposed to light, and other data. These seeds can also be used in demonstra- 

 tions 3, 4, and 5. 



SUPPLEMENTARY READING: 

 See Demonstration A-l. 



DEMONSTRATION A-3: Effect of duration of imbibition period (soaking) on 



effectiveness of a given light exposure in promoting germination of 



light-sensitive seeds. 



MATERIALS: 



1. Light-sensitive seeds such as Grand Rapids lettuce or peppergrass (Lepidium virginicum). 



2. Eight petri dishes or plastic sandwich boxes, with lids. 



3. Two to four thicknesses of white or colorfast blotters, filter paper, or paper towels. 



4. Eight black sateen cloth bags made of two layers of cloth large enough to hold the dishes. As an alternate 

 method dishes can be placed between the folds of a large, double layer of black cloth, or they can be 

 wrapped with two layers of aluminum foil. 



PROCEDURE: 



1. Prepare the dishes as described under demonstration A-2. Use 0.2 percent KN0 3 instead of tapwater 

 for peppergrass. 



2. All dishes except dish 2 are immediately placed in darkness in the black cloth bags or between folds of 



