Ill 



LIVING MATTEE 



towards the light that conies in at the windows (positive heliotaxis), 

 while the surfaces of the leaves spread out perpendicularly in the 

 direction of the light rays (transverse heliotaxis). In plants with 

 aerial roots these turn and grow towards the darkest part of the 

 room (negative heliotaxis). 



FIG. 21. Bacterium photometrii'iim, in micro-spectroscope. (Engehnanii.) The bacteria 

 are collected in the region of the ultra-red and yellow rays. 



Heliotactic movements are especially favoured by the blue and 

 violet rays ; red and yellow rays are practically inactive (Fig. 22). 



Loeb (1888) described phenomena of heliotaxis in many 

 animals, which are perfectly comparable with those observed on 

 plants ; they are also determined by the most refrangible rays of 

 the spectrum. The mechanical explanation of the phenomenon is, 

 according to Loeb, that the symmetrical points of an organism 



FIG. 22. (lalium, aparina showing heliotaxis. (Noll.) The plant curves left or light, to the 

 source of light, as indicated by the arrow L. The leaves exhibit transverse heliotaxis. 



possess equal excitability, and the unsynimetrical points unequal 

 excitability ; the points nearest the buccal pole possess an excit- 

 ability greater than, or different in form from, that of the points 

 nearest the opposite pole. By this is meant that with unilateral 

 illumination the muscles of the excited side are thrown into a 

 tension which is relatively greater or less than that of the muscles 

 of the opposite side, so that the animal deviates in the direction of 

 its movements, in the sense of positive or negative heliotaxis. In 



