June 23, 1905.] 



SCIENCE. 



941 



Photosynthesis is a topic ^^;hieh has re- 

 ceived a full share of physiological investi- 

 gation throughout the past century; yet 

 the problems demanding attention are too 

 numerous for complete enumeration. The 

 mechanism of gaseous exchange in leaves 

 has repeatedly been experimentally proved 

 to be the function of the stomates. After 

 critical physical experimentation, Brown 

 and Escombe have recently reported that 

 the results of their studies of diffusivity 

 through multiperforate septa are closely 

 applicable to the herbaceous leaf with its 

 stomates and substomatic chambers. As- 

 suming their calculations to be correct, and 

 granting that all of the incoming carbon 

 dioxide is removed, it is estimated that with 

 the stomates open the maximum observed 

 rate of fixation of CO2 in HeUanfJtus (which 

 is .134 c.c. per square centimeter per hour) 

 would be only 5.2 to 6.3 per cent, of the 

 theoretical capacity of the diffusion appa- 

 ratus of the plant. In other words, with a 

 gradient between the outer and inner air 

 of only 5 to 6.5 per cent, pressure, the 

 maximum observed fixation is well ac- 

 counted for. 



Important problems in the general study 

 of photosynthesis may well begin with that 

 of a better knowledge of the structure of 

 the chloroplasts and the constitution of 

 chlorophyll. Neither of these, however, is 

 absolutely essential to further physiological 

 observations of a fruitful kind. One of 

 the questions long ago raised is still perti- 

 nent : what is the connection between 

 chlorophyll and the plastid in which it 

 is embedded? An answer to this question 

 may perhaps afford in time an answer 

 to the general inquiry as to the location of 

 the true photosynthetic property. If 

 chlorophyll is always the same chemically, 

 it is perhaps probable that the first product 

 of photosynthesis may always be the same, 

 although this is not necessarily true. In 



any ease, the chief problems hinge upon the 

 method of decomposition of carbon dioxide 

 and water and the synthesis of the first or- 

 ganic product. Neither the h\T)othesis of 

 Bayer, Erlenmeyer, Crato, Bach, Putz, nor 

 any other, has, to any considerable de- 

 gree, been made capable of experimental 

 proof, although that of Bayer has been 

 most generally accepted. Each of these 

 assumptions offers some suggestions for 

 future work. Perhaps it may as well 

 be said that they, to a certain extent, 

 bias future research. Nevertheless, even 

 when the chemical reactions in this syn- 

 thesis become known it may yet remain 

 problematical how the energy of sunlight, 

 that is, of those rays most absorbed, with 

 wave lengths of 660 to 680 /j. is made avail- 

 able, or whether it is this energy directly 

 or indirectly which is concerned in the de- 

 composition. It has been Avell assumed 

 that the light waves may not be immedi- 

 ately serviceable, but only after transfor- 

 mation into other forms of energy. Fur- 

 ther, it is not known to what extent this 

 energy is operative in subsequent trans- 

 formations. The conditions under which 

 photosynthesis occurs have been worked out 

 with a fair degree of accuracy, the status 

 of these problems having been well set 

 forth by Ewart and others. It is known 

 that when deleterious agents act at a given 

 concentration merely to inhibit the as-sim- 

 ilatory function (the cell not being perma- 

 nently injured) there is no evident change 

 in the chlorophyll, from which it has been 

 inferred that the assimilatory arrest has its 

 origin in the plasmatic stroma. In all 

 cases photosynthesis can not long proceed 

 except under conditions of health of the 

 protoplasts. Nevertheless, the effects of 

 deleterious agents have not always been 

 studied by very delicate tests, and further 

 attention might be bestowed upon this mat- 

 ter by the use of the photobacterial method, 



