EFFECTS UPON GERMINATION 817 



of the rays actually reaching the protoplasm was proportional to the 

 quanta they carried, and that the rays acted in accordance with the Ein- 

 stein principle for photochemical reactions. On the basis of these results, 

 Kommerell believed that only light reaching the living portions of the 

 seed was effective and that the action was photochemical. She ran simi- 

 lar experiments on the light-inhibited seeds of Phacelia tanacetifolia but 

 found that the low light intensities which forced Nicotiana and Lythrum 

 seeds did not inhibit Phacelia seeds. 



Kommerell 's methods have opened the way for exact experimentation 

 in this field. The fact that the effectiveness of light falling on the seeds 

 was proportional to the wave-length is certainly significant, but it is 

 doubtful whether her main conclusion is justified by the facts. Einstein's 

 principle was deduced for an ideal, reversible, unimolecular reaction. 

 There is little probability that the effect of light on seeds is so simple. 

 Kommerell's conclusion appears still more questionable when it is recog- 

 nized that Einstein's principle does not always hold for simple chemical 

 reactions to which it was supposed to apply. In some cases (91, page 332) 

 the photochemical equivalent is less than that demanded by this princi- 

 ple, and in other reactions it is many times greater. 



We have yet to learn to what extent light modifies germination by 

 acting upon the seed coats, on the one hand, and upon the protoplasm, on 

 the other, and just how the various rays bring about such modifications. 



DOSAGE OF LIGHT REQUIRED 



The dosage of light necessary to modify the germination of seeds and 

 fruits varies greatly with different species. It has already been pointed 

 out that Poa achenes respond only to high light intensities; in fact, direct 

 sunlight gives optimum results. Chloris achenes with hulls intact also 

 require rather high intensities for maximum germination and are more 

 favored by continuous illumination than by alternate light and dark 

 periods. In the use of alternate light and dark periods, it is beneficial 

 to have illumination during the first period because of the readiness with 

 which these achenes become "dunkelhart." Viscum album fruits, 

 strict light-obligates, need relatively high light intensities for germination 

 and are killed by continuous darkness. With the light-inhibited seeds 

 of Phacelia tanacetifolia a relatively high intensity of light is necessary to 

 give noticeable inhibition, and high intensities must be applied continu- 

 ously to give fullest inhibition. 



On the other hand, some light-favored seeds need only a very small 

 dosage of light to stimulate germination. Raciborski (86) found that 

 1 hr. exposure of imbibed tobacco seeds to weak diffuse light followed 

 by darkness gave complete germination within 48 hr. The extreme 

 sensitiveness of tobacco seeds to light may account for disputes in the 

 literature as to the need of light for their germination. Exposure of 



