268 COMPENDIUM OF GENERAL BOTANY. 



nomena of double refraction, I will add the critical observation 

 of Schwendener. The existence of unequal or one-sided molec- 

 ular tensions in the smallest particles of the membrane (the 

 micellae) cannot be assumed, because we cannot conceive of 

 them as having points of fixation. 



3. The arrangement of the axes of the optically active ellip- 

 soids of elasticity of the cell-membrane always coincides with 

 morphologically definable directions : one axis is always radial ; 

 the other two lie in the tangential plane, of course extending in 

 different direction. The radial axis is usually the shortest. 



4. The shortest optical axis of elasticity always coincides 

 with the axis of maximum swelling ; least swelling is in the di- 

 rection of the optical axis of greatest elasticity. If the micellae 

 are of unequal dimensions in the different directions and are all 

 surrounded by equally thick layers of water, it will be readily 

 seen that the expansion must be less parallel to the longest axis 

 of the micellge than to the shortest axis. 



III. TEMPEKATUEE, LIGHT, GEAVITY, AND OTHEK 

 FACTOES, IN THEIE EELATION TO PLANT-LIFE. 



A. EFFECTS OF TEMPERATURE. 



The discussion of the effects of temperature and light will in 

 general be based upon the results of the investigations of NAGELI, 

 SACHS, PFEFFER, and FRANK. 



(a) Production of Warmth and Cold. 



Processes productive of heat and cold occur within the plant. 

 As a result, plants give evidence of a subjective temperature ; 

 that is, under certain conditions their temperature is different 

 from that of the surrounding medium. Eespiration, that is, the 

 conversion of hydrogen- and carbon-bearing compounds into 

 CO 2 and H 2 O, produces a rise in temperature. In an extreme 

 case (flowers of Aroidece) the rise in temperature may be 15 C. 

 Daring the germination of barley the temperature also rises, as 

 is well known. Evaporation of moisture from the plant tends to 

 reduce the temperature. 



