Problems of Plant Physiology 47 



A stimulus may be positive at one intensity and negative at another. 

 Geotropism is positive for most roots although moisture conditions may 

 reverse this reaction. Most stems are positively heliotropic but in Linaria 

 this undergoes a natural reversal since the young flower stems are 

 positively heliotropic, becoming negative as they elongate (52, p. 318). 

 Agapanthus and Papaver are similar examples. 



Questions of symbiosis and metabiosis also arise. Just as numerous 

 questions presented themselves concerning photosynthesis, we also have 

 the field of chemosynthesis with many unsolved problems. Sugar may 

 constitute a product both in constructive and in destructive metabolism 

 and hence a physiological and chemical classification will not agree (48, 

 p. 262). Questions have arisen concerning the wave length of light 

 best for green plants. Pfeffer gives it as between Frauenhofer's 

 lines C and D of the spectrum and of 660-680 uu wave length (1, Bd. 

 I, p. 330). Reinke's (53) spectrophore can displace diffraction grating 

 in certain experiments of this kind. The growing of lichens on windows 

 of houses and their mechanical action shows unusual capabilities (54). 



The prevention of flower wilting by the use of PbNOs deserves 

 further study (55). The change of color of flowers due to a change of 

 temperature, as above referred to, has a partial parallel in the 

 enantiotropic substance, mercury iodide, and a further striking parallel 

 in the double salt Cu.. L 2Hgi which changes color from red to brown 

 and to red again on cooling (56). Though not analogous the sudden 

 darkening of the pulvinus of Mimosa after stimulation shows that water 

 has taken the place formerly occupied by air (57). A continued study 

 of Schumann, Roentgen and other rays as well as different colored 

 lights in various combinations of temperature and gases would net 

 further valuable results. 



Certain problems concerning the chemical action of different plants 

 need further attention as that of wood destroying fungi; the penetra- 

 tion of structures by bacteria as egg-shells (58) ; the reported corrosion 

 of stone, mollusk shells, oyster shells, and oolitic iron by algae (41, 1920, 

 Bd. II, p. 360) ; and the penetration of membranes by fungal filaments 

 (59). The unusual reported case of algae growing on a painting for 

 about 200 years shows the capabilities in certain directions (60). Of 

 the halogen group iodine and bromine are found in marine plants, as 

 would be expected and probably in land plants (1, Bd. I, p. 433) though 

 the latter is contradicted by Fresenius (61, p. 542). Other questions 

 arise on this point. Lead is often present in plants but a practical 

 method for its volumetric estimation is lacking according to Fresenius 

 (62) and Sutton (63) advises weighing for its accurate determination. 

 The chemist can assist in the many plant problems by determining 

 substances in plants, but he cannot tell what ones and the amounts 

 necessary for the proper functioning of the metabolic processes. This 

 the plant physiologist must determine by actual experiment. 



Facts in physiology once established are often disbelieved or 

 forgotten. For example, the neglected work of Ingenhousz on the obtain- 

 ing of carbon by green plants was afterwards brought to light by Liebig; 

 the water-culture method first used by Woodward was subsequently 



