VIII] OR CELL-AGGREGATES 399 



its quadrangular portion subdivide by periclines, and the triangular 

 portion by oblique anticlines (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 

 considerations point, but a widespread and important factor in the 

 morphology of the cryptogams. But meanwhile, without en- 

 quiring further into this compUcated 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 into an ellipsoid form, and may finally extend 

 into a cylinder with rounded or ellipsoid end. 



This latter very simple case is illustrated in the development 

 of a pollen-tube, where the rapidly growing cell develops into the 

 elongated cylindrical tube, and the slow-growing or quiescent part 

 remains behind as the so-called "vegetative" cell or cells. 



Just as we have found it easier to study the segmentation of 

 a circular disc than that of a spherical cell, so let us begin in the 

 same way, by enquiring into the divisions which will ensue if the 

 disc tend to grow, or elongate, in some one particular direction, 

 instead of in radial symmetry. The figures which we shall then 

 obtain will not only apply to the disc, but will also represent, in 

 all essential features, a projection or longitudinal section of a solid 

 body, spherical to begin with, preserving its symmetry as a solid 

 of revolution, and subject to the same general laws as we have 

 studied in the disc*. 



* In the following account I follow closely on the lines laid down by Berthold ; 

 Protojtlasmamechanik, cap. vii. Many botanical phenomena identical and similar 

 to those here dealt with, are elaborately discussed by Sachs in his Physiology of 

 Plants (chap, xxvii, pp. 431-459, Oxford, 1887) ; and in his earher papers, Ueber 

 die Anordnung der Zellen in jiingsten Pflanzentheilen, and Ueber Zellenanordnung 



