132 The Phenomena of Morphogenesis 



ends is often evident. That the cytoplasm is the seat of this polar differ- 

 ence is shown by the fact that, when vacuolate cells divide, the first 

 indication of the plane of division, and thus of the polar axis, is the ap- 

 pearance of a cytoplasmic diaphragm in the position where the future 

 partition wall will be formed (p. 25). In such cells the direction of the 

 axis may be related to gradients in hormone concentration, oxygen, or 

 other factors. This polar difference may be visible in the contents of the 

 cell, for in Enteromorpha ( Muller-Stoll, 1952), in Isoetes (Stewart, 1948), 

 and other plants the chromatophore is almost always on the side of the 

 cell away from the base of the thallus, or plant body. The distribution of 

 chloroplasts in higher plants is also sometimes polar. 



ED EI 



B 



X 



PHLEUM 



mmMmsm 



B 





1 



w SPOROBOLUS 



Fig. 6-10. Polarity in root-hair development. A, B, and C, successive stages, with root 

 apex toward left. In Phleum, the last division is unequal, and the cell toward the apex 

 forms a root hair. In Sporobolus, the division is essentially equal, and the cell toward 

 the apex does not always form a root hair. ( From Sinnott and Bloch. ) 



The wall itself may show polar behavior, a fact which is of particular 

 importance in producing differences in cell shape. Most cells are nearly 

 isodiametric at the beginning, and if one at maturity is much longer than 

 wide, this is the result of more rapid growth in length. Such differential 

 growth, in turn, presumably comes from differences in the fine struc- 

 ture of the wall, which itself is ultimately dependent on factors in the 

 cytoplasm. Wilson (1955) has shown that in the wall of the large cells 

 of the alga Valonia there are two systems of orientation of cellulose 

 fibrils which converge to two poles at the ends of the cell. The complex 

 and remarkable shapes of many cells, both in simple organisms and 

 within the tissues of larger ones, are probably due to a complex pattern 



