440 



LECTURE XXVIT. 



of the medullary rays are simply nothing further than the expression of the general 

 rule that the anticlinal cell-walls are the orthogonal trajectories of the periclines. 

 Supported by this general rule it is also possible in any given transverse section of 

 wood, the annual rings of which are known, to register at once the course of the 

 medullary rays; or, when the latter are more distinctly recognisable than the 

 annual rings, to make out the converse. 



In these cases we have had to do with processes of growth where increase 

 in volume takes place in an easily intelligible manner only at the margin or 

 circumference, the cells there formed, however, undergoing no further growth worth 

 mention, although they pass over into the definitive condition of permanent tissue, 

 in which the second phase of growth— i. e. elongation — is suppressed. 



We will now, however, turn our attention to those cases, at first of the simpler kind, 

 where an organ consisting entirely of embryonic tissue grows throughout its entire 



mass ; where increase in volume 

 takes place not merely at the cir- 

 cumference but also in the interior, 

 and accordingly division-walls ap- 

 pear also in the interior. For 

 the facilitation of the problem 

 we will even here only concern 

 ourselves with transverse and lon- 

 gitudinal sections, or structures 

 naturally fiat, because the con- 

 sideration of proper stereometric 

 relations would not only cause 

 great prolixity, but can also be 

 dispensed with in the meantime 

 for our purpose. The simplest 

 case is of course presented by 

 P,(- 2^j fiat organs which consist of a 



single layer of cells only. 

 Our further considerations will gain in clearness, and will be much facilitated 

 if we construct for ourselves on paper in advance various possible cases. It is 

 already clear from what has been said so far that the ordinary case consists in the 

 rectangular intersection of the periclines and anticlines. We may thus, for example, 

 imagine an elliptical disc (Fig. 275) consisting of embryonic substance, and propose 

 to ourselves the question, In what direction are the division-walls formed in it, 

 if the growing surface (which always remains elliptical) is cut up into a large 

 number of cells by periclinal and anticlinal division-walls intersecting at right 

 angles? Now the problem is solved according to geometrical principles in the 

 elliptical figure given. In the first place, the area is divided into four quadrants by the 

 major and minor axes of the ellipse, and the two foci are marked//! As periclines, 

 two other ellipses, P and p, are drawn, possessing the peculiarity that their foci are 

 likewise situated in //; or, in other words, the three ellipses drawn are confocal. 

 Instead of three we might draw a large number of confocal ellipses. In order 

 that the anticlines to be drawn in the figure everywhere cut the periclinal ellipses at 



