Observations: 
Record the date of exposure to light, the subsequent date of germination 
and the extent of germination. Moist peppergrass seeds exposed to light (as 
in pot 1) will germinate in 3 to 4 days after exposure. Seeds covered with 1 ~ 
centimeter of soil will not germinate since they are in the dark (as in pot 2). 
Slitting the soil with a knife blade exposes some of the buried seeds to light. 
Thus (as in pots 3 to 6) germination of seeds occurs in the slit made in the 
soil. 
The soil may shrink away from the sides of the pot and expose some seeds 
to light and seedlings may appear. Avoid this by planting the seeds away 
from the edge of the pots, Slitting the soil at regular intervals after planting << 
illustrates that germination will occur any time the seeds are exposed to | 
light. Slitting the soil, in effect, simulates field cultivation. Thus, cultiva- 
tion, while destroying plants and seedlings, also brings weed seeds such as 
peppergrass to the surface of the soil, where they receive light, germinate, 
and produce more weeds. 
Supplementary Reading: 
Koller, Dov. Germination. Sci. Amer. 200: 75-84, April 1959. 
Toole, E. H., Borthwick, H. A., Hendricks, S. B., and Toole, V. K., 
Physiology of seed germination. Ann. Rev. Plant Physiol. 7:299-324. 1956. 
U.S. Agricultural Research Service. New light on plants. U.S. Dept. Agr., 
Agr. Res. 1: 3-5. 1953. 
U.S. Agricultural Research Service. How light controls plant development. 
U. S. Dept. Agr., Agr. Res. 8: 3-5. 1959. 
DEMONSTRATION A-2: How to test various kinds of seeds to determine their 
light requirement for germination. 
Materials: 
1. A minimum of four petri dishes or plastic sandwich boxes with lids. 
2. Ordinary color-fast or white blotters, filter paper, or paper towels. 
3. 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. 
4. Seeds of several kinds of weeds. (Although some kinds of seeds are 
known to be light-sensitive, many kinds have never been tested. This is 
especially true for weed seeds, so they would be the more interesting 
group to investigate.) 
Procedure: 
1. Collect seeds of several kinds of local weeds. In general, seeds will re- ! 
tain their viability fairly well when stored dry ina refrigerator. Some ~| 
suggested seeds known to be light-sensitive are peppergrass (Lepidium 
virginicum and L, densiflorum), henbit (Lamium amplexicaule), and 
hedge mustard (Sisymbrium officinale), Other weed seeds worthy of 
investigation are shepherds-purse (Capsella bursapastoris), yellow 
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