CHAPTER 7 



SALT RELATIONS OF VASCULAR 



PLANTS 



I am indeed not unaware that this path is obscured by 

 clouds which will pass over from time to time. Yet 

 these clouds will easily be dispersed when it is possible 

 to make the fullest use of the light of experience. For 

 Nature always resembles herself although she often 

 seems to us, on account of the inevitable deficiency of 

 our observations, to disagree with herself. 



C. Linnaeus. 



A. Zones of Ion Absorption in Roots 



When excised or attached roots are placed for a short time in a 

 solution of salts containing a radioactive ion, it is often observed 

 that a zone near the tip becomes more radioactive than do more 

 mature regions (Fig. 38a). Radioactivity is not, however, confined 

 to the extreme root apex, and whereas a region of low activity 

 frequently coincides with the zone of cell elongation, there is often 

 a region of high radioactivity in the root hair zone (Fig. 38b). Such 

 experiments indicate that different regions of the root retain salts to 

 differing extents, but do not, as has sometimes been thought, provide 

 much information as to the most active zone of absorption. The 

 low concentration of absorbed salt in the zone of elongation, for 

 example, could be attributed to concomitant growth by uptake of 

 water, or to transport of salts away from this region rather than to a 

 slow rate of absorption. Wiebe and Kramer (1954) attributed the 

 accumulation of salts near the tip of roots to the high metabolic 

 activity in this region, and to the absence of conducting tissue (see 

 Fig. 3a, p. 15). They demonstrated that only a small percentage of 

 various ions absorbed by the terminal 4 mm of barley roots is 

 translocated elsewhere (Table 8) whereas as much as 34 per cent of 

 the phosphate taken up was transported away from one of the more 



111 



