394 PLANT PHYSIOLOGY 



loids of the embryo. It is therefore desirable to keep seeds in 

 storage under conditions where metabolism will continue at the 

 slowest rate possible. Cool, dry places fit this requirement. The 

 seeds of Salix pierotii generally lose their viability within a week, 

 but when kept in an ice box over dilute sulphuric acid and oc- 

 casionally aerated, 53% may germinate at the end of a year. Pol- 

 len also retains its viability longer when kept dry. 



It would be advantageous if one could tell the viability of a 

 seed sample without planting some of it and waiting for time 

 to tell. The most popular of the various methods tried is the cat- 

 alase test. Many researches have tended to show that the more 

 viable a seed is, the more catalase activity it possesses. Although 

 this test is at best a relative one, it has exceptions. In Pinus 

 strobus the seed with the higher catalase activity germinated the 

 poorest (Knight, 1931). Also it is possible to kill seeds without 

 appreciably affecting their catalase activity. Davis (1925-1926) 

 found that dead seeds when soaked in warm water lost their cat- 

 alase activity very rapidly, so that a more successful test can be 

 made by comparing the loss in activity after soaking. If a seed 

 loses 0.9 of its catalase activity after soaking, its viability is low. 

 This is, consequently, a good attempt to make an absolute test 

 for a relative condition. The reducing activity of seeds has also 

 been used as a criterion of their vitality, but this method has the 

 same objections as the catalase test. 



While reproduction is of the highest importance in the produc- 

 tion of seeds and fruits, the foregoing chapter has attempted to 

 touch only a few of the broader problems in their general physio- 

 logical aspects, leaving to the agronomist, the horticulturist, and 

 the geneticist the development of those phases most closely con- 

 nected with their special problems. 



QUESTIONS 



1. Why do seeds fail to germinate in wet, muggy soil? 



2. Where would you expect a higher percentage of blossoms in the sum- 

 mer, — in the tropics or the arctics? Why? 



3. How does one compute the viability of seed from the percentage curve? 



4. What is the effect of the size of the seed upon the size and growth of 

 the resulting plant? 



5. Transeau (1916) found that in Spirogyra the normal length of the vegeta- 

 tive cycle varied inversely with the surface area of the cells. How might Kraus 

 and Kraybill explain this? 



