46G Profs. Percy Frankland and Marshall Ward. 



at temperatures otherwise suitable for growth when light rays at the 

 blue-violet end of the spectrum act on the growing cells or the 

 spores. 



Unless it can be shown that the high temperature kills the organism 

 by acting on its food-medium outside the cell, the above is an argument 

 against any such simple explanation of the action of the blue- violet 

 rays, especially since experiments with other plants point to similar 

 destructive actions of such rays in cases where no question of a bath- 

 ing food-solution can be raised unless we choose to regard the sap 

 in the vacuole of a living cell as such a bathing medium, as indeed it 

 is, in a sense. It would probably, however, seem a strange proposal 

 in the present condition of plant-physiology to refer the inimical 

 actions of light solely to reactions of the cell sap though the 

 possibility could perhaps not be denied. 



(2.) The second external factor to be considered is light. 



The experiments show beyond all cavil that light-rays of higher 

 refrangibility bring about the death of the spores at all temperatures 

 worth consideration ;* in this case the curve of growth does not 

 come into account. The evidence also shows that these rays depress 

 the otherwise normal curve; but the difficulties begin here, because 

 we have no means of expressing the intensity of the light used in 

 terms similar to those used in reference to temperature. 



It is proved that a light of low intensity, passing through screens 

 which transmit only blue-violet rays, kills the young filaments at low 

 temperatures, which in the absence of these rays does not injure the 

 filaments at all. It is also proved that these light-rays at even 

 higher and more favourable temperatures, seriously retard the growth 

 of more advanced filaments, so that their curve is much more de- 

 pressed than the curve of similar filaments at the same temperature 

 but protected from ihe blue-violet rays. 



When the temperatures ai-e very favourable to growth, however, it 

 is often difficult to determine quantitatively the effect of the light- 

 rays on the curve of growth, because the latter can only be observed 

 for a period too short for the effect of the light action to be measur- 

 able ; even in these cases, however, the curve is frequently seen to 

 be commencing its depression towards the end of the observation 

 period, and the cultures exposed to the light are seen to be deficient 

 in crop or in spore-production subsequently. 



At temperatures above the optimum it is extremely difficult to 

 judge of the damage due to the light apart from injury due to the 

 temperature, but the general conclusion seems likely that high tem- 

 peratures act so much more rapidly than the light that most of the 

 . disastrous effects are due to the former. 



* I.e., it is no use discussing temperatures known to be dangerous on their own 

 account. 



