PLANT PHYSIOLOGY PRESENT PROBLEMS 129 



known that external conditions may considerably modify the de- 

 tails of spindle formation, and perhaps other details in nuclear and 

 cell-division. The important point in every case is to determine if 

 the same physiological purpose may be accomplished. 



It is extremely important, however, to the subject of physio- 

 logy that the methods which have made possible these cytological 

 advances shall be extended and utilized in developing a knowledge 

 of all of the various activities of the cell. In this way, a clearer 

 insight may be given of many abstruse metabolic processes; and 

 certainly further light may be thrown upon the matter of protoplas- 

 mic decompositions and secretions, the production of enzymes and 

 alkaloids, tannins and other products. Going hand in hand with 

 observations upon fresh material, the limitations of micro-chem- 

 istry alone should determine the possibilities in this direction of the 

 work. 



In such cytological investigations, Fischer's work on the artifi- 

 cial production of effects resembling those seen in fixed protoplasm 

 should be borne well in mind. This work is timely, and may assist 

 in checking irrational developments b}^ forcing a proper regard for 

 a comparison of the effects observed in fixed tissues with those 

 shown by the living material. 



There are, moreover, but few directions in which the study of 

 metabolism and metabolic products may not profit from cytologi- 

 cal research. A notable instance of what there is to be done is well 

 indicated by the work of the late Dr. Timberlake on the division 

 of plastids and the development of the starch grain. 



Photosynthesis is a topic which has received a full share of 

 physiological investigation throughout the past century; yet the 

 problems demanding attention are too numerous for complete enu- 

 meration. The mechanism of gaseous exchange in leaves has repeat- 

 edly 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 herb- 

 aceous leaf with its stomates and substomatic chambers. Assuming 

 their calculations to be correct, and granting that all of the incom- 

 ing carbon dioxide is removed, it is estimated that with the stomates 

 open the maximum observed rate of fixation of C0 2 in Helianthus 

 (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 accounted for. 



Important problems in the general study of photosjmthesis may 

 well begin with that of a better knowledge of the structure of the 



