Some Seeds Use Light; Others Manage Without 



All seeds carry phytochrome, and many plants use it to get out of 

 the ground. Phytochrome in some seeds needs scarcely more than a light 

 flash. Seeds of some of the worst weeds are in this class. Farmers make 

 use of a photoperiodic principle when they follow advice not to cultivate 

 after applying a pre-emergence herbicide. These weed-killers do not 

 penetrate soil deeply, but neither can deep-buried weed seed make a 

 start without light. Turning soil up and under can give these seed light 

 aplenty to bring up a weed crop. On the other hand, there are plant seed 

 that require more light to assure germination. 



Still other kinds of plants would not exist if their buried seed depended 

 on light. They survive and thrive in a light-controlled world because 

 nature has allowed an emergency hook-up in the seeds' growth -starting 

 mechanism. Once out of the ground, these seed plants comnnonly follow 

 conventional photoperiodic channels and behavior. Scientists emphasize 

 that evasions of photoreaction in plant growth are unusual enough to be 

 the proverbial exceptions that "prove the rule." Understanding a non- 

 conforming plant is like understanding the normal individual from the 

 behavior of abnormal ones. 



Seed of a plant such as tobacco normally depend on light to start 

 germination, but can manage without light provided temperature gives 

 a go-signal. The right alternations between coolness and warmth aid 

 these seed to sprout in darkness. 



Seed of crops like corn regularly sprout without their light-signaling 

 pigment. Although the seed receive light at planting time, the phytochrome 

 does not react because the seed are dry. They germinate in darkness, 

 evidently by aid of favorable moisture, temperature, and other soil con- 

 ditions. 



UGHT AS PART OF THE MECHANISM 



Sunlight 



Since the sun pours red and far-red light at one time on plants, this 

 raises a question: Is some difference in sunlight an original cause of 

 earth's vegetational growth changes? 



It is evident that when bright sunlight strikes plants in the open, red 

 overinfluences the far red and pushes phytochrome's dual-form atoms 

 toward concentration in active form. A forest cover, on the other hand, 

 may absorb so much of sunlight's red that far red becomes the ruling 

 influence. 



In any case, when light containing both red and far red strikes phyto- 

 chronne, there is some competitive pushing of the reversible atoms, which 

 are always in motion. This is in contrast to the one-way push toward 

 concentration during a dark photoperiod or under artificial light limited 

 to red or far red. 



16 



