PLANT PHYSIOLOGY PRESENT PROBLEMS 139 



for calcium in this proteid compound, there results a lessening of the 

 capacity for imbibition, attended by unfavorable consequences. 

 It has been further ascertained by the work of May, Kearney and 

 Cameron, Kusano, Aso, and others, that there is for each plant a 

 certain more or less definite relation between calcium and magnesium. 

 Nevertheless, further experimental proof is needed before this 

 brilliant hypothesis may be acceptable in its entirety. It may here 

 be noted that in a paper read by the writer before the Society for 

 Plant Morphology and Physiology it is indicated that magnesium 

 compounds exert upon the marine algae the least injurious action of 

 all nutrient bases. On the other hand, it has not been demonstrated 

 that the marine algae require calcium. 



The general phenomenon of chemotaxy, or chemotropism, de- 

 mands searching investigation in view of the recent work of Jen- 

 nings on flagellates, that of Newcombe on root responses, and other 

 studies on the fungi. There is much to be done in determining the 

 effects of heat and cold upon special processes, in a study of the 

 relations of temperatures to other conditions of the environment, 

 and in showing the limitations of accommodation phenomena. 

 In the latter study, moreover, the effects of accommodation upon 

 the general constitution of the organism should be followed. Stimu- 

 lation at high or low temperatures merely expresses an intensified 

 or modified irritability. It may be observed in this place that death 

 at the supramaximal or subminimal may be due to changes of a 

 very definite nature; but, as Vines has indicated, this means very 

 little. To say that death at the supramaximal is due to the coagu- 

 lation of an albuminoid as suggested by Kuehne is insufficient. 

 For the immediate effect upon the protoplasm of this high tem- 

 perature must also be of consequence. The external conditions of 

 temperature of the effects of a modification of conditions are more 

 or less readily determinable; but it has not been possible to follow 

 the internal changes which result. It may be noted that the freezing- 

 point of a plant is lower than that of the expressed sap; yet of 

 course the freezing-point is not necessarily a valuable indicator of 

 injury. The effects of temperature upon reproduction will be treated 

 of later. 



The symbiotic relationship of fungus and root to Mycorhiza offers 

 a fine opportunity for careful investigation. The studies which have 

 already been made serve only to put the reader in a state of hopeless 

 confusion. 



The universal phenomenon of irritability as manifested by trophic 

 phenomena has been a fruitful field of investigation. The general 

 methods of irritable response have been determined; and the best 

 work of such investigators as Haberlandt, Noll, Czapek, Newcombe, 

 MacDougal, and others has more recently been directed to the deeper 



