Polarity 135 



are not isolated. In bits of tissue cut from Enteromorpha, for example, 

 Miiller-Stoll (1952) found that the cells near the apical portion of the 

 piece regenerate papilla-like structures but that the cells at the base 

 form rhizoids. 



To prove the existence of polar behavior in the cells of one of the 

 higher plants is more difficult. Here it is sometimes possible to re- 

 generate a new plant from a single cell or small group of cells, especially 

 in the epidermis, but it is not easy to relate the polarity of the newly 

 produced structure to that of the cell from which it grows. 



POLARITY IN ISOLATED CELLS 



In many cases, polarity may be studied in cells that are isolated in 

 nature and not through experiment. The most notable example of this 

 is the egg of the rockweed, Fucus, which is discharged into the water and 

 there is fertilized and grows into a new plant. The Fucus egg is com- 

 parable to the eggs of certain animals that develop in water and that 

 have proved such a rich source of knowledge of early embryology. More 

 work has been done on this egg than on any other naturally isolated plant 

 cell. 



The unfertilized egg is naked, and its nucleus is at the center of the 

 cell. It shows at the beginning no polarity whatever nor is there any 

 visible differentiation in its cytoplasm. After fertilization, the egg falls 

 to the bottom and in about 12 to 24 hours, under normal conditions, a 

 protuberance appears on its lower surface. This develops into a rhizoid by 

 which the young plant becomes anchored to the bottom. Soon the egg 

 divides in a plane at right angles to the axis of the protuberance. The 

 two cells that result are very different in shape and in their future de- 

 velopment. The upper, rounded cell gives rise to the main portion of the 

 thallus. The lower one forms little besides the rhizoid. The growth of 

 the rhizoid and the first division of the egg establish a permanent polar 

 axis in a system which at first is quite without one. Here is evidently one 

 of the simplest expressions of polarity among plants. 



Among earlier investigators of the Fucus egg were Kniep ( 1907 ) , 

 Nienburg (1922a and b), Lund (1923), and others. The more recent 

 experiments of Whitaker and his colleagues have provided a large body 

 of detailed information. Only the main facts will be presented here. This 

 work has been reviewed by Whitaker ( 1940 ) and Bloch ( 1943 ) . 



Gravity seems not to be an important factor in the induction of the 

 polar axis for, if the eggs are kept in the dark, the rhizoid develops in 

 any direction. There is evidence that in eggs of the related Cystosira 

 barbato, if reared in darkness, the rhizoid is formed at the point of en- 

 trance of the sperm ( Knapp, 1931 ) . Light is clearly a very important 



