VIII] OF THE GROWING POINT 633 



is not a resultant of the growth of the cells individually considered*. 

 Such asymmetry of growth may be easily imagined, and may 

 conceivably arise from a variety of causes. In any individual cell, 

 for instance, it may arise from molecular asymmetry of the structure 

 of the cell-wall, giving it greater rigidity in one direction than 

 another, while all the while the hydrostatic pressure within the 

 cell remains constant and uniform. In an aggregate of cells, it 

 may very well arise from a greater chemical, or osmotic, activity 

 in one than another, leading to a locahsed increase in the fluid 

 pressure, and to a corresponding bulge over a certain area of the 

 external surface. It might conceivably occur as a direct result of 

 preceding cell-divisions, when these are such as to produce many 

 peripheral or concentric walls in one part and few or none in another, 

 with the obvious result of strengthening the boundary wall here 

 and weakening it there ; that is to say, in our dividing quadrant, 

 if its quadrangular portion subdivide by perichnes, and the 

 triangular portion by oblique antichnes (as we have seen to be 

 the natural tendency), then we might expect that external growth 

 would be more manifest over the latter than over the former areas. 

 As a direct and immediate consequence of this we might expect 

 a tendency for special outgrowths, or "buds," to arise from the 

 triangular rather than from the quadrangular cells; and this turns 

 out to be not merely a tendency towards which theoretical con- 

 siderations point, but a widespread and important factor in the 

 morphology of the cryptogams. But meanwhile, without enquiring 

 further into this complicated question, let us simply take it that, 

 if we start from such a simple case as a round cell which has divided 

 into two halves or four quarters (as the case may be), we shall at 

 once get bilateral symmetry about a main axis, and other secondary 

 results arising therefrom, as soon as one of the halves, or one of 

 the quarters, begins to shew a rate of growth in advance of the 

 others; for the more rapidly growing cell, or the peripheral wall 

 common to two or more such rapidly growing cells, will bulge out, 

 and may finally extend into a cylinder with rounded end. This 

 latter very simple case is illustrated in the development of a 



* Sachs, Pflanzenphysiologie {Vorlesung xxiv), 1882; cf. Rauber, Neue Grund- 

 legungen zur Kenntniss der Zelle, Morphol. Jahrb. vni, p. 303 seq., 1883; 

 E. B. Wilson, Cell-lineage of Nereis, Journ. Morph. vi, p. 448, 1892; etc. 



