PHYSIOLOGICAL CIL^RACTERISTICS OF AXIATE PATTERNS 87 



has a polarity at the time of discharge from the thalkis that may be effec- 

 tive in the absence of sufficiently intense external differential after dis- 

 charge. The rhizoid outgrowth appears on one side of the cell before the 

 first division, and the plane of the first division is normally at right angles 



B 



Fig. 26, A-C. — Three developmental stages of the alga, Fiicus. A, growth of rhizoid pre- 

 ceding first division; B, two-cell stage, rhizoid gradient; C, later stage, thallus and rhizoid 

 gradients. 



to the direction of rhizoid outgrowth (see Fig. 26, A, B). The pole oppo- 

 site the primary rhizoid becomes the apical pole of the thallus. No definite 

 death gradient has been observed in the eggs before determination of po- 

 larity by light or some other external factor; but when the rhizoid begins 

 to grow out, it becomes a region of high susceptibility, and death pro- 



