EXTERNAL CAUSES OF GROWTH AND FORMATION. I 311 



dorsiventrality in higher plants also, e. g. in Thuja (FRANK, 1873), and Begonia 

 (ROSENVINGE, 1889). In most cases it is possible by altering the direction of 

 the light-incidence to produce corresponding changes in the dorsiventrality; 

 thus fern prothalli may be made to form sexual organs on the upper side by 

 illuminating the lower, but the dorsiventrality in Marchantia is so fixed by 

 heredity that new growths are no longer affected by external factors, but follow 

 in their development the plan of structure already laid down by the fully- 

 developed regions. 



We must content ourselves with these examples and in conclusion inquire 

 as to the significance of the quality of light, the colour or wave-lengths of the 

 rays. We arrive at this most important fact that in growth and formative 

 processes the more refrangible rays are those which are operative. In many 

 cases it has been shown that the less refrangible rays which do most of the work 

 in carbon-dioxide assimilation act morphogenetically like darkness. We see, 

 therefore, how little the cessation of carbon-dioxide assimilation has to do with 

 etiolation, since green plants become etiolated in red light in spite of assimilation 

 taking place at the same time. The formation of chlorophyll goes on equally 

 well in light of all wave-lengths (REINKE, 1893). [Other pigments behave 

 differently ; in Oscillaria, according toGAiDUKOw's (1903) experiments which 

 wait confirmation however a so-called complimentary adaptation of colour 

 takes place, that is to say, the cells become red in blue light and blue in red light.] 



For long it was believed that ultra-violet light had quite a special effect 

 on the external formation of the plant. SACHS'S (1887) statements on the point 

 have not, however, been confirmed (compare p. 364). The influence of Rontgen 

 and of other recently-discovered rays, has been studied, but without leading 

 to results of any physiological value. [Compare KORNICKE, 1905.] 



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