Terminology and the Transport Equation

Were it the case that auxin transport takes place within a plant part—a stem, a petiole, a root—by travelling in a solution of constant concentration (density) in a continuous stream of constant velocity (speed), then, the efficiency of the 

Concentration in this equation means the mass of auxin per volume of the transporting system just as area means the cross-sectional area of the system. The area would be identical with the cross section of the plant part examined if auxin were moved across the total area of all cells of the tissue. It is quite certain that this is not the case it is, however, uncertain what part of the tissue is involved in transport (see Sect. 3.3.6). Thus the quantities area and concentration are as yet not accessible to experimental determination. However, in the product area x concentration the area of the unknown transport channels is eliminated. It remains a quantity with the dimension mass per length which means the amount of mobile auxin present in unit length of the transport system, identical with unit length of the plant part. This quantity, which can be estimated experimentally, was termed capacity by van der Weij (1932). 

Van der Weij compared the transport system with a conveyor-belt and used the term capacity (Kapazitdt) to describe the load of a unit length of this belt ( die Kapazitat. d.h. die Stoffmenge, welche jedes cm des Trans-portbandes enthalten kann, van der Weij 1932, p 482). This expression has led to some confusion in the literature. The term capacity has been used, misleadingly, in the sense of transport intensity or maximum intensity. Since it means length density , the term transport density has been proposed, and this term will be used in this article (Kaldewey 1968 a). 

the transport process may be described by the following equation  

The term velocity instead of rate of movement is used intentionally in the transport equation. The expression rate of movement of auxin has been used by some authors to mean the distance moved in unit time. and by others to mean the amount passing through the tissue in unit time The term rate has therefore become incurably ambigu- 

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