1920] HURD—FUCUS SPORES 



43 



rhizoidal cell. In other words, the direction of the first protuber- 

 ance is at right angles to the first cleavage plane, whatever the 

 direction from which it is subsequently illuminated. After the 

 first cross-wall has formed in the rhizoid, however, a change in 

 the direction of illumination results in an angular bend at the tip. 



Group orientation 



The preceding experiments have demonstrated the power of 

 light to establish the polarity of germinating spores of Fucus 

 injlatus. Yet another factor was found to exert an orienting: 



influence on the spindle no less potent than that of light, that is, 

 the proximity and direction of other germinating spores. Rosex- 

 vinge (19) described this most striking and interesting phenomenon 

 in other species of Fucus and in Ascophyllum. The first cross- wall 

 forms perpendicular to the direction of the adjacent spore or 

 group of spores, or, if the spore be one of a group, perpendicular 

 to the direction of the center. The cell toward the interior of the 

 group, or toward the source of the stimulus in the case of more 

 isolated cells, becomes the rhizoidal cell (fig. 2). For want of a 

 better term I have called this phenomenon group orientation. 

 It is best studied in cultures germinated in darkness, and hence 

 free from orienting effects of light. 



This phenomenon is as conspicuous in groups of 2, 3, or 4 eggs 

 as in masses of 50 or 100, so long as they are within the distance of 

 each other through which the stimulus is effective. This distance 

 is usually 0.2-0.3 mm., but occasionally spores as much as 0.5 

 mm. apart have shown the mutual orienting influence. If there 

 are only two spores concerned, the first cleavage planes are often 

 parallel, and the rhizoids, growing from the inner cells, meet tip to 

 tip. In the small groups of five or six the rhizoids, all growing 

 toward the interior, make rather symmetrical starlike designs. 

 In the larger groups or masses of spores the phenomenon is made 

 evident by the fact that no rhizoid is ever found taking a direction 

 away from the group. Although the finding of many groups of 

 eight lying together just as they escaped from the oogoniai sac, and 

 beautifully oriented with respect to each other, suggests an inherent 

 polarity established by the relative positions of the eggs in the 



