482 



LECTURE XXIX. 



If tlie growing-point is situated at the free end or apex of the organ, then 

 the base of the latter is its oldest part, and every transverse section meets a part 

 so much the younger the nearer it lies to the apical growing-point. If, however, 

 the embryonic tissue which represents the growing-point is situated at the base 

 of the organ, as is the case in many leaves, internodes, etc., then the relative age is 

 in the reverse direction. 



The longitudinal direction does not suffice to fix the position of the axis of 

 growth in an organ. To define this more exactly the transverse section of the organ 

 in question must be taken into consideration : in the transverse section of any 

 organ there is a point which, with respect to the outline and the anatomical structure, 

 constitutes the organic centre. Every small portion of the surface of the transverse 

 section exhibits in its structure a side turned towards the periphery, and one turned 

 towards this organic centre, and is bounded laterally by lines which we may term the 



FIG. 318.— Transverse section of the woody body of the stem of a L 



radii of the transverse section. Just as the organic centre by no means necessarily 

 coincides with the geometric centre of the transverse section, so also the radii are not 

 always straight, but often curved lines, as is clear at once from the consideration of 

 Fig. 318. Very generally the internal structure of the organ exhibits peripheral 

 layers and radial arrangement at the same time, as is distinctly perceived on any 

 transverse section of wood ; but every hair also, and even petioles and leaves, as well 

 as the transverse section of single cells, may be thus regarded. 



If we now suppose the organic centres of all the transverse sections of an organ 

 (a shoot-axis, the mid-rib of a leaf, or a root, etc.) to be connected together by a 

 line, this is the longitudinal axis, and also the axis of growth of the organ; and, 

 according to circumstances in each case, it may be a straight or a curved line, or 

 if at first straight it may subsequently become curved, and conversely. A plane 



