218 



PLANT PHYSIOLOOY 



made rapidly and at once, else changes of considerable ex- 

 tent occur. Within two minutes after the darkened leaf is 

 put on the microscope, the chlorophyll grains have gone 

 halfway to the day position (a). 



The behavior of the plate-like chromatophores of the alga 

 Mesocarpus is similar.* When intense light falls upon a 



.Figure 18. Cells of Mesocarpus Sp. 

 a — cell with chromatophore in diffuse daylight position, 

 b — chromatophore bending in too intense light, 

 c — chromatophore bent to reduce absorption of light. 



. Mesocarpus cell, the chromatophore presents only its edge 

 to the incident rays, but when the light is less intense the 

 chromatophore bends, as in c, so that the greater part 

 of it presents its profile to the light. Still less intense yet 

 strong light induces the position indicated in b, and light 

 of only moderate intensity or less is indicated by the chro- 

 matophore being expanded at right angles to the incident 

 rays as in a. The chromatophores change their positions 

 only with a change in the direction or the intensity of the 

 illumination, but under favorable conditions the changes 

 can be accomplished rapidly. 



Whether heliotropism and heliotaxis are or are not funda- 

 mentally similar phenomena there is no experimental evi- 

 dence for determining. The effect of light upon the cell is 

 not precisely known, though it is probable that it chemi- 



* Oltmanns, F. 1. c, p. 209. 



