2. Prepare six lots of 100 seeds each and distribute each lot of seeds uniformly over the surface of the -oil 

 in each pot. Treat the pots as follows: 



(a) Pot 1 — leave the seeds on the surface. Do not cover them with soil 



(b) Pots 2 to 6 — Cover the seeds with 1 centimeter of soil. Level the soil surface and tamp gently. 



3. Do not water the top surface of the soil. Place all the pots in the large gla^-s dish or enameled pan ar. 

 irrigate the soil in the pots by adding water to the dish. Maintain the pots in this dish, being careful to 

 have them standing in about 1 centimeter of water at all times 



4. Place all pots in unfiltered sunlight. 



5. Keep each pot covered with a glass square, at least until the seedlings that will develop are well • 

 lished. The soil will be kept moist by capillary action. The glass cover will admit light but will prevent 

 excessive water loss from the soil and maintain a high humidity at the soil surface. This is important 

 during the critical periods of germination and early seedling growth. 



6. Write the name of the plant material, the date of planting, and the date of treatment on a label, and 

 insert it into the soil at the edge of the pot. 



7 . Treatments : 



(a) Pot 1— See 2(a). 



(b) Pot 2— See 2(b). 



(c) Pot 3— Immediately after covering the seeds with soil, with a knife make a narrow slit 3 to 4 centi- 

 meters deep in the soil across the diameter of the pot. 



(d) Pots 4, 5, and 6 — 1, 2, and 4 weeks after planting, respectively, repeat the process described for pot 3. 



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 frons 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, cultivation, 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: 



Borthwick, H. A. Light effects with particular reference to seed germination. Internatl. Seed Testing 



Assoc. Proc. 30: 15-27. 1965. 

 Roller, Dov. Germination. Sci. Amer. 200: 75-84. April 1959. 

 Toole, E. H., Hendricks, S. B., Borthwick, H. A., 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 He- 1: :> 5. 1953 

 U.S. Agricultural Research Service. How light controls plant development. U.S. Dept. Agr., Agr Res v 



3-5. 1959. 



DEMONSTRATION A-2: How to test various kinds of seeds to determine their 

 light requirement for germination. 



MATERIALS: 



1. Seeds of several kinds of weeds. (Although some kinds o( seeds an 1 known to be light-sensitive, many 

 kinds have never been tested. This is especially true for weed seed-, so they would be the more interesting 

 group to investigate. Seeds of many commonly grown small-seeded ornamentals and grasses exhibit light 



requirements and might also be included in this demonstration ) 



2. A minimum of four petri dishes or plastic sandwich boxes, with lids. 



