488 



Mr. F. Darwin and Miss D. F. M. Pertz. [May 30, 



instance of what we have observed in other cases, viz., that tertiary- 

 roots bend downwards more readily than upwards. It is interesting 

 because it is a point of resemblance between them and diageotropic 

 secondary roots. The slowness of the growth of the tertiaries in this 

 experiment was probably due to the development of more than one 

 secondary root. 



Fig. 4 shows the same state of things, namely, that the direction of 

 growth of the sections marked 2 is different on the two sides. Thus 

 section 2 is directed obliquely upwards in the right-hand tertiary roots, 

 and downwards on the left. A similar contrast exists between the 

 sections marked 3. These changes of direction are due, as in fig. 3, to 

 changes in the position of the trough. 



In the later experiments, of which fig. 5 is an example, the tracings 

 were made each day in a different coloured paint, so that a complete 

 record of all tertiaries was obtained. This was not the case in fig. 4, 

 so that some of the sections are necessarily left without numbers. 



In fig. 5, the numbers 1, 2, 3, etc., give the tracings made on 

 successive days. It will be seen that in the younger part of the root, 

 near the lower end of the figure, the sections of the tertiary roots do 

 not begin with the numerals 1 or 2, but with 5, 6, or 7 ; this is because 

 when sections 1 and 2 were traced, the lower (younger) tertiaries had 

 not yet appeared. 



After section 1 was traced, the trough was tipped up at an angle of 

 39° (right side highest), and remained in that position while 2 and 3 

 developed. This accounts for the bend downwards in the oldest root 

 on the right (E i), and the slight upward bend in L ii (section 3). 



The trough was now reversed, being placed at 19° (left side highest) 

 and being allowed to remain in that position while sections 4 and 5 

 developed. It will be seen that the curvature of the sections 3, 4, 5 

 is upward on the right (except in root E iv) and downwards on the 



Fie. 4. 



left. 



