50 



Growth 



case of radial divisions in the phellogen, Bouygues (1930) concludes that 

 pressure is not a factor. 



The plane of division is evidently related to the polarity of the cell and 

 is further discussed under this topic (p. 131 ). It has been studied particu- 

 larly in the egg of Fucus. Here centrifugal force, light, electricity, and 

 gradients in concentration of various substances have been found to affect 

 this plane. 



In certain colonial blue-green algae and flagellates, in pollen mother 

 cells, and in some other cases where division in all the cells is simul- 

 taneous and in the same plane, the planes of each successive division tend 



o° 



45° 



CL 



90° 





0° 



45° 

 103 



90° 



Fig. 3-17. Distribution of angles between mitotic spindles and longitudinal axis of the 

 ovary in an elongate type of cucurbit fruit ( above ) and an isodiametric one ( below ) . 

 There is evidently a higher proportion of divisions nearly at right angles to the axis 

 (spindles with low angles) in the former. In the latter, divisions are approximately 

 equal at all angles. (From Sinnott.) 



to be at right angles to each other so that a regular pattern of cells in 

 twos, fours, eights, sixteens, and so on, all in one plane, is produced. Be- 

 tween divisions the cells tend to grow but not enough to make them 

 isodiametric, so that the next division is at right angles to the longer axis 

 of the cell, as it would be in a least-surface configuration (Geitler, 

 1951). Division in three planes sometimes occurs, producing cubical 

 colonies. 



In many instances there is no obvious explanation for the particular 

 plane in which a cell divides, and we are forced to attribute this to genetic 



