MEASUREMENT OF GROWTH IN LENGTH. 545 



pushed forwards by the transverse zones which are beneath it on the shoot-axis. The 

 movement of the shoot-bud in space is the sum of the elongations which the transverse 

 sections of the shoot-axis beneath the bud are simultaneously undergoing. This is 

 more especially of interest in the case of the roots: in their case also the so-called apical 

 growth is a meaningless expression, for the growing-point covered with the root-cap 

 is driven forwards passively, by the elongation of the older transverse zones of the 

 root which lie behind it. In this case, however, the length of the growing portion 

 is remarkably short, only 2-10 mm.: this, however, appears really to be of advantage 

 when the object is to drive the root-apex forward in the soil. The root behaves in 

 this respect like a nail being driven into solid wood by the strokes of a hammer. 

 Where this regard for the mechanical conditions of the forward movement of the 

 apex of the root is not necessary, we find that the relations of growth are 

 different even in roots. In the long aerial roots of Aroids, and of a species of 

 Ct'ssus, I found ^ by measurements that the elongating region situated behind the 

 growing-point is of considerable length, amounting to several centimeters, as in 

 the case of shoot-axes ; this is evidently because it would here be superfluous to 

 concentrate the force which drives the growing-point forward a few millimeters behind 

 the latter, as is necessary in the case of roots growing in the resisting earth. 



Instead of the transverse lines mentioned so often, it is also possible in some 

 cases to make use of certain natural marks for observing the distribution of growth, 

 provided of course that there is sufficient ground for the assumption that these natural 

 marks are equidistant at first, and also stop at equal distances finally. Thus, 

 Askenasy^ employed the leaf- whorls of the Characeae, or, what amounts to the same 

 thing, the lengths of the internodes, in order to gain information as to the alterations 

 in space and time of the growth, from the growing-point to the parts fully developed. 

 In the same way it is possible in the case of any other chosen stems with numerous 

 internodes (which, however, must finally attain equal lengths) to find the stages of growth 

 of the individual internode to a certain extent severally represented in the then existing 

 stages of growth of all the internodes which are still elongating. It is obvious that 

 such an experiment will not be possible if the plant is of such a nature that the 

 various internodes attain diff"erent lengths, or differ in any other way as to their 

 relations of growth. 



In the case of organs the growing-point of which terminates in an apical cell 

 which forms either one series of segments by transverse walls, or two or three series of 

 segments by oblique divisions, the length of the surface lying betw^een each two 

 segment-walls may be similarly employed, like the space lying between two artificially 

 made transverse lines. Provided (and this is actually realised in most cases,) that 

 the successive segments have equal lengths originally, the process of growth may 

 be judged from the relative lengths of the segments lying one over the other. Under 

 certain circumstances, in the case of very simple organs which consist of one cell 



* I described the abnormal behaviour of aerial roots respecting the length of the growing region 

 in Arb. des bot. Inst, in Wzbg., B. I, p. 593 (1874). 



2 The ingenious treatise of E. Askenasy, depending on careful reflection, 'Über cine neue 

 Methode, um die Vei-thcihmg der Wachsthumsinfensität in zvachsenden Thcikn zu bestimmen,' is 

 in the Verhandlungen des naturhistor.-med. Vereins zu Heidelberg, N. S. II, 2 II. (Winter). Unfor- 

 tunately I have not space to go further into the contents of this treatise. 



[3] Nn 



