136 The Phenomena of Morphogenesis 



factor in Fucus (Hurd, 1920). In eggs lighted from one side by white 

 light of a certain intensity (or light of particular wave lengths), the 

 rhizoid always forms on the side opposite the source of light, and the 

 first wall is laid down at right angles to this direction. Nienburg ( 1922a, 

 b) showed more specifically that it is not the direction of light but the 

 intensity gradient that is the determining factor. 



Lund ( 1923 ) was able to prove that the first division wall in the Fucus 

 egg was at right angles to the flow of an electric current and that the 

 rhizoid grew toward the positive pole. Here the polar axis can evidently 

 be determined electrically. 



A peculiar phenomenon first noted by Rosenwinge ( 1889 ) but studied 

 particularly by Whitaker is the so-called "group effect." If an egg of 

 Fucus lies near a group of other eggs, its rhizoid will develop toward this 

 group ( Fig. 6-13 ) . Whitaker observed that this occurs even when the eggs 

 belong to different species. These results have been attributed to the 



&-^-:>. : .*':''i7 Fig. 6-13. The "group effect" in Fucus eggs. Where 

 there is a cluster of these, the rhizoidal pole is typically 

 on the side toward the other eggs. ( After Whitaker. ) 



establishment, in the medium near the egg, of a concentration gradient 

 of metabolic products from the other eggs, but Jaffe (1955) finds evi- 

 dence that such a gradient, if it exists, does not involve H ion, COo, or 

 2 . Jaffe also found (1956) by exposing eggs to polarized light that the 

 rhizoids tended to develop in the plane of polarization. 



Whitaker ( 1937 ) subjected Fucus eggs to centrifugal force and showed 

 that in such cases the rhizoid grows from the centrifugal pole. Polarity 

 here seems to be dependent on the rearrangement of materials in the egg. 

 Other factors, such as pH, temperature, auxin, and even the shape of the 

 egg have also been shown by Whitaker to modify egg polarity. 



Not many other cases of the induction of a polar axis in isolated algal 

 cells are known, although in certain green algae the polarity of the young 

 plants developing from swarm spores is determined by the way in which 

 these spores become attached to the substratum ( Kostrum, 1944 ) . 



The remarkable umbrella-shaped alga Acetabularia is really a single 

 cell though it may be several centimeters tall. During its development it 



