SALT RELATIONS OF VASCULAR PLANTS 



123 



(v). When transpiration is stopped in a particular leaf or leaflet, 

 the amount of salt delivered to that structure is drastically curtailed, 

 as was demonstrated in a simple experiment with Vicia faba plants 

 (see Plate II facing p. 122). 



Care must be taken to distinguish between the rate at which an 

 individual ion is transported over a given distance, and the total 

 quantity of salt moved in a given interval of time. The former 

 depends on the linear rate of mass flow, and its measurement is 

 affected by the sensitivity of technique employed. The more 

 sensitive the method of detection, the more rapid the apparent rate 

 of transport. The total quantity of salt transported on the other 



(a) 



(b) 



Fig. 40. Diagram illustration transport of objects on a mechanical conveyor 

 a. Objects supplied at a rate of 1 per sec; endless belt moving at the rate 

 of 1 ft/sec; delivery at the rate of 1 per sec; b. Object supplied at the same 

 rate as in a; belt moving at the rate of 6 ins /sec; delivery is at the same rate 

 as in a, but the belt becomes more heavily loaded. 



hand, is a function of the rate of flow of the solution by volume, 

 and the concentration of salt contained in it. Comparison with a 

 mechanical conveyor will make the distinction clear (see Fig. 40). 

 If a single article is placed on the machine, its rate of transport will 

 depend on the rate of movement of the conveyor belt, but if a 

 number of articles are supplied the rate of delivery will ultimately 

 depend on the rate of supply, and will be independent within limits 

 of the rate of movement of the belt. If the belt moves more slowly 

 it will merely become more heavily loaded than when it moves 

 quickly (Fig. 40b). It is only when the rate of movement of the 

 belt is so slow that it is unable to accept articles at the rate supplied, 

 or when it stops completely, that the speed of the conveyor controls 

 the rate of delivery. 



A similar situation arises in the transport of salt by mass flow 



