PLANT PHYSIOLOGY PRESENT PROBLEMS 131 



similar to the curve of relation between temperature and respira- 

 tion. There is a certain maximum for each temperature. It has also 

 been ascertained that there is a certain economic light intensity 

 beyond which there is no increased photosynthetic activity, and 

 doubtless only injury. This is of special interest in connection with 

 some recent work by Weis. Recognizing the facts that plants are of 

 very different types with relation to their light requirements, he has 

 sought to get an expression of their assimilatory energy. He finds 

 that (Enothera biennis, a well-marked sun plant, fixes under favor- 

 able conditions of temperature, and in direct sunlight, about three 

 times as much CO 2 as in diffuse light (light of one sixtieth to one 

 ninetieth this intensity). On the other hand, Poly podium vulgar e 

 assimilates in diffuse light somewhat more energetically than in 

 direct, while Marchantia polymorpha occupies a position intermedi- 

 ate. This will be welcomed by physiologists as a field for whole- 

 some ecological study, for an extension of such investigations to an 

 analysis of plant associations with relation to the light factor may 

 yield profitable results. 



In 1901, Freidel made the surprising report of success in securing 

 outside of the living plant a gas exchange similar to the photosyn- 

 thetic action of chlorophyll. He was later unable to confirm his 

 previous conclusions, nor were the subsequent results of Macchiata 

 and Herzog concordant. Recently, Molisch has employed upon this 

 problem the photobacterial method of Beijernick. He finds that 

 the expressed sap of certain plants may for a time maintain photo- 

 synthetic activity, but since usually the sap loses this power when 

 filtered through a Chamberlain filter, it is believed to be due to the 

 presence of living plasmatic particles. Nevertheless, it is claimed 

 that an exchange of gases characteristic of photosynthesis may pro- 

 ceed in a solution of the leaves of Lamium album dried crisp at 35 

 C. and then "rubbed up" in water and filtered. The observation 

 demands much further study, for it must be remembered that the 

 test is by means of the liberation of oxygen, and Ewart has shown 

 that some bacterial pigments may have the power, of evolving 

 oxygen. In the last-named case the gas evolved appears to be, as 

 he states, " occluded oxygen absorbed from the air by the pigment 

 substance excreted by the bacteria." 



It cannot be stated at the present time, however, as was assumed 

 from FreidePs first work, that there is any enzyme concerned in the 

 photosynthetic activity. 



To a large extent the problems involved in a study of the assimi- 

 lation of nitrogen are limited by the very imperfect chemical know- 

 ledge of nitrogenous products, and may not, therefore, be very 

 clearly defined. Practically, the whole question of the formation 

 of amides, proteids, or other nitrogenous compounds in plants 



