448 



GENERAL PHYSIOLOGY 



open surface of their gorgeous blossoms to the sun, and in this 

 position follow its slow movement throughout the day ; at evening 

 their blossoms have a direction almost the opposite of that in the 

 morning. In many plants, as Stahl ('85) has shown in the horse- 

 tails, the direction of growth is influenced by light in a very 

 interesting way even in the spore ; in the division of the spore-cell 

 the first division-wall, which divides the cell into two parts, is 

 formed at right angles to the direction of the incident rays of 

 light. A characteristic difference in the kind of phototaxis of 

 the two halves is noticeable, such that the rhizoid-cell, from which 

 the roots develop later, is always turned away from the source of 

 light, and the prothallium-cell, from which the parts above the 

 earth are derived, toward the source (Fig. 222). 



Among animals the investigations of Loeb ('90) and Driesch 

 ('90) in recent times have likewise demonstrated wide-spread 



phototactic phenomena. But, although 

 it is not altogether easy to obtain a 

 correct view of these phenomena in 

 the cell-community of the plant, it is 

 much more difficult in the complex 

 community of the animal body, on 

 account of the varied share taken by 

 the sense-organs, the nervous system, 

 the motile organs, etc. Hence it is 

 advantageous here to turn our attention 

 primarily to the simplest relations, such 

 as exist in the free-living cell. 



The phototactic phenomena of uni- 

 cellular organisms, observed by Priestley 

 and Ehreriberg, were followed out more 

 fully by Nageli, Hofmeister, Baranetzky, 

 Stahl, Klebs, Cohn, and other botan- 

 ists, but the fundamental labours of Strasburger first gave an 

 exact picture of the laws of the phenomena. 



Strasburger (78) made his investigations chiefly upon swarm- 

 spores of various chlorophyllaceous Algce, and observed their 

 behaviour toward light falling from a window upon one side of a 

 suspended drop. Essentially the same phenomena were shown 

 by flagellated swarm-spores of very different species. The be- 

 haviour of the swarm-spores of Ulothrix may serve as a type. In 

 diffused daylight of slight intensity these small flagellated cells 

 hasten in straight paths to the edge of the drop that is turned 

 toward the light, and collect there in great crowds. If the in- 

 tensity of the light be increased, which Strasburger accomplished 

 by bringing the preparation nearer the window or employing 

 direct sunlight, with a certain intensity the swarm-spores begin to 

 leave the positive side of the drop, i.e., the side that is turned 



IG. 222. Division of the spore- 

 cell of a horse-tail under the 

 influence of light. The arrow 

 indicates the direction of the 

 rays. a, Position of the 

 division-wall, b, direction of 

 the mitotic figure. (After 

 Stahl.) 



