LECTURE XXIX. 



AXIS OF GROWTH; POLARITY; LATERALITY; RELATIONS 

 OF POSITION. 



The terms at the head of this chapter denote a series of relations in space 

 within the plant, the consideration of which we cannot here entirely pass over, 

 because they afford us clear ideas on certain general processes of growth ; and it 

 is shown at the same time that a series of the most important relations between 

 growth and external influences — i.e. phenomena of irritability — are only intelligible 

 if we aljstract from the relations of form evident to the senses, and keep in view 

 these relations which are only accessible to abstraction. 



On every organ it is easy to distinguish two opposite ends, which we may 

 designate base and apex \ The base is the place where the organ arises from its 

 parent-organ, and is fixed to it ; the other end, the apex, on the contrary, is free 

 and movable. As the organ elongates, it is the apex which is pushed forwards in 

 space by means of the growth taking place between it and the base. When the 

 bud at the end of a shoot, or the growing-point of a root, gradually traverses a path 

 of several centimetres, or even of several metres, in a perpendicular, horizontal, or 

 oblique direction, this is accomplished by means of the growth between apex and 

 base, by the intercalation of new substance, and, simply because the base is fixed 

 while the bud with the apex is free, it is the latter which is moved forwards in space. 



A line which is supposed to run in the interior of the organ from the base to 

 the apex gives the longitudinal direction, or the direction of growth in length ; and 

 a plane which takes in this line is a longitudinal section. Every surface standing at 

 right angles to this plane is a transverse section. 



' In my ' Lehrbuch der Botanik'' (Anfl. II. 1S70, § 26) I have already referred sufficiently in 

 detail to the significance, not only morphological but also physiological, of the distinction between 

 apex and base of the organs of plants ; and I pointed out in my treatise ' Über Stoff und Form der 

 Pflanzenorgane' (Arb. des bot. Inst, in Wzbg., B. II, pp. 452 and 689) that Voechting had subse- 

 quently, in his book ' Über Organbildung im Pflanzettreich'' (1878), given another inaccurate mean- 

 ing to the polarity designated by base and apex. In my text-book, as well as in my later treatise, 

 ' Über die Anordnung der Zellen in jüngsten Pflanzentheilen' (Arb. des bot. Inst, in Wzbg., B. II, 

 p. loi), the previous entirely inaccurate definition of the axis of growth by Hofmeister was also 

 corrected. 



[3] Xi 



