386 Profs. Percy Frankland and Marshall Ward. 



3. Further testing of the measurements and curves on growing fila- 

 ments under like conditions, confirms the confidence in their accuracy, 

 and they may be accepted as very good approximations. 



4. Some of these measurements bring out clearly the extreme sen- 

 sitiveness to changes of temperature of the growing filaments, and 

 emphasise clearly how difficult it is to avoid this source of error. 



5. The mode of action of the light may be conceived of in several 

 ways, keeping in view the differences of effect on spores and filaments. 

 First, we might suppose it promotes oxidations in the surrounding 

 food materials, resulting in the formation of poisonous substances 

 which kill the spores but not the filaments ; this (taking into 

 account the resistance of the spores to physical agencies, and the 

 evidence previously given) seems unlikely, for unless the living proto- 

 plasm of the actively growing cells has some extraordinary power of 

 destroying such poisons as fast as they are made, whereas the dormant 

 protoplasm of the resting spore is incapable of this, it seems incredible 

 that the spores, otherwise so highly resistant, should succumb more 

 easily than the otherwise so slightly resistant filaments. Secondly, 

 we might suppose that the light action takes effect directly on some 

 easily destroyed reserve material in the spore, which does not exist as 

 such in the actively metabolising growing cell. This would explain 

 the retardation of germination, or the death of the spores, according 

 to the amount of destruction of the spore contents, very well ; but 

 it is not easy to accept the assumption that the light is totally with- 

 out effect on the growing cell. A third possibility seems to be that 

 the light action makes itself effective in promoting some intense 

 metabolic activity in the spores and growing cells alike, and which 

 is connected with enhanced respiration. In this case we might 

 suppose the spores to suffer from the too rapid consumption of 

 their unstable reserve materials (as before), while the growing cells 

 do not show the effects so long as plenty of food material is still avail- 

 able in the hanging drop, and, therefore, so long as the filaments 

 are still measurable. At a later period, however, the overworked 

 machinery results in the production of much feebler plants, capable 

 of developing a few poor spores only, or even none at all, such as was 

 observed in several cases where the cultures were allowed to go on. 

 Here, then, we can understand why the direct measurements often 

 give no decisive results : the actively growing filaments can only be 

 measured during a short period, but the cumulative effect of the mal- 

 nutrition is not evinced by diminished growth until after the measure- 

 ments have ceased, or at least till towards the end of the period.* 



6. But if the latter hypothesis is accepted, we have to recognise 



* This is decidedly against action on the food materials, since one would expect 

 the effect to make itself evident very early in the growth as the injurious bodies 

 reach the sensitive cells. 



