634 THE FORMS OF TISSUES [ch. 



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

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

 remains behind as the so-called "vegetative" cell qf 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 study 

 in the disc*. 



(1) Suppose, in the first place, that the axis of growth lies 

 symmetrically in one of the original quadrantal cells of a segmenting 

 disc; and let this growing cell elongate with comparative rapidity 

 before it subdivides. When it does divide, it will necessarily do 

 so by a transverse partition, concave towards the apex of the cell : 

 and, as further elongation takes place, the cylindrical structure 

 which will be developed. thereby will tend to be again and again 

 subdivided by similar transverse partitions (Fig. 280). If at any 

 time, through this process of concurrent elongation and subdivision, 

 the apical cell become equivalent to, or less than, a hemisphere, 

 it will next divide by means of a longitudinal, or vertical partition; 

 and similar longitudinal partitions will arise in the other segments 

 of the cylinder, as soon as it comes about that their length (in the 

 direction of the axis) is less than their breadth. 



But when we think of this structure in the solid, we at once 

 perceive that each of these flattened segments, into which our 

 cyUnder divided to begin with, is equivalent to a flattened circular 

 disc; and its further division will accordingly tend to proceed like 



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

 Protoplasmamechanik, 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 earlier papers, Ueber 

 die Anordnung der Zellen in jiingsten Pflanzentheilen, and Ueber Zellenanordnung 

 und Wachsthum {Arh. d. botav. Inst. Wurzbnrg, 1877/78). But Sachs's treatment 

 differs entirely from that which I adopt and advocate here: his explanations being 

 based on his "law" of rectangular succession, and involving complicated systems 

 of confocal conies, with their orthogonally intersecting ellipses and hyperbolas. 



