Polarity 131 



ity and that, as wood elements differentiate, polarity is gradually im- 

 pressed upon them. Cells inversely oriented are now unable to unite, 

 and the translocation of materials in them, tending in each to follow the 

 original direction of flow, is seriously disturbed. This gradual assumption 

 of polarity is perhaps related to changes at the cell surface as the wall is 

 formed or in the structure of the wall itself. 



Another manifestation of polar activity in histological characters, per- 

 haps related to the basipetal tendency in the renewal of cambial activity 

 or to the polar flow of auxin (p. 384), may be observed in the reconstitu- 

 tion of severed vascular strands across the ground parenchyma of pith 

 or cortex in herbaceous dicotyledons (Simon, 1908; Sinnott and Bloch, 

 1945; Jacobs, 1954). This always begins at the basal end of a severed 

 strand and proceeds downward toward the apical end of the cut bundle 

 or to uninjured ones. 



Cell Polarity. To test Vochting's contention that polar behavior of a 

 tissue is the result of the polarity of its individual cells is not easy. The 

 fact that very small tissue pieces retain their original polarity and that 

 inversely grafted tissues do. not fuse supports Vochting. Many other facts 

 can also be cited. The two daughter cells following a division are often 

 unlike (p. 133). In these cases, each of the two types is found invariably 

 on the same side, toward or away from the tip of the axis. Thus in many 

 young roots the last division of the surface cells is unequal, the smaller 

 daughter cell becoming a trichoblast and producing a root hair (p. 190). 

 This cell is always on the side toward the tip of the root. Before division, 

 the apical end of the mother cell is also more densely protoplasmic. In 

 some cases (Phleum) the division is markedly unequal and polar. In 

 others (Sporobolus) the two cells are more nearly equal and a root hair 

 is not always formed (Fig. 6-10). Here the polar behavior is much less 

 marked. In the leaf epidermis of monocotyledons some cells divide 

 unequally, and the one toward the leaf tip becomes a stomatal mother 

 cell. These facts suggest that the cells themselves have a polar orienta- 

 tion. 



The tendency of cells to divide in specific directions is at the bottom of 

 all form determination, since it is concerned with the plane of division 

 and thus the direction of growth. In the growth of elongate gourd fruits, 

 for example, divisions are predominantly at right angles to the axis of 

 the fruit, but in isodiametric ones they are at all angles (p. 51). Whether 

 polarity is a quality of the whole developing organ or simply of its 

 component cells is still uncertain and is a problem involving the deeper 

 one of the relation between cell and organism. Various examples of 

 polarity in unequal cell divisions have been discussed and figured by 

 Bunning (1957; Fig. 6-11). 



Even when the cell does not divide, the difference between its two 



