No. 512] PRESENT PROBLEMS IN PL A XT ECoLuay I'M 



at our disposal a recording instrument whose importance 

 can not be overestimated. Because of its directly appli- 

 cable data, its slight cost and ease of manipulation, it may 

 well take precedence in comparative habitat investi- 

 gations. 



The greatest desideratum in the way of habitat study 

 now seems to be an instrument for the measurement of 

 light values. It seems probable that the effects of the 

 varying light intensities and qualities in the habitat are 

 generally underestimated at the present time. The com 

 parative studies already made with actinometers and 

 photometers are suggestive, but the value of the data as 

 indicative of relative light conditions from the standpoint 

 of vegetation is open to serious question. Thus far 

 photographic papers have been most generally used. 

 Aside from the high percentage of error attending the 

 matching with standards of the colors developed, these 

 papers are almost wholly affected by the most refrangible 

 rays of the visible spectrum. Plant functions in general 

 find their optima among the least refrangible rays. Ow- 

 ing to selective absorption, a decrease in the intensities of 

 one portion of the spectrum does not necessarily imply 

 a similar decrease throughout. The objection urged 

 against photographic paper also holds for the use of other 

 substances which develop precipitates or gas pressure 

 under the influence of light. Unless papers can be de- 

 vised which have an increased range of sensitiveness and 

 which can be used in connection with color screens, the 

 continuance of actinometric measurements in habitat- 

 seems almost a gratuitous pursuit. 



Naturally the spectro photometer has been suggested 

 for overcoming this difficulty. By means of photographic 

 methods, in which a diffraction grating is used for the 

 disj MM -ion-piece and photographic plates of known sensi- 

 tivity to the various light rays, more accurate results 

 might be obtained. The difficulties of manipulation, how- 



