Organismal Capillaries in the Plant Transport Systems

The basic transport unit of Fig. 1 will now be generalized to encompass the diverse medium- and long-distance pathways and then used for describing transport in the network by assigning F(HK) values to the different tissues. 

In the basic transport unit of Fig. 1, Mq and Mi were required to be permeable to and water and to enable H to enter them and move along their internal surfaces by the specific transport mechanism described in Section 3.1. In Section 3.5, the apoplasm between Mq and Mq was also assumed to show proton conduction. Thus two xylem conduits with their walls in contact with each other constitute, together with the pits between them as the organismal capillaries, a variation of the basic transport unit. The wall phase without the pits may be considered as having an appreciably lower conductance for H , K , and water, while the pits form 

As described in Section 2.1, the xylem conduits communicate not only with each other but also with the tissues around them. A number of lateral transport systems abut against the axially arranged xylem conduits, which will not be described here. However, the possibility of a lateral movement of the xylary sap will be utilized below for the qualitative description of the transport of water in plants. 

The transport system of the sieve tubes is also a multiplet of generalized transport units (pTU) with the sieve plate pores as organismal capillaries. Though the sieve tubes are thin as such, the dependence of the hydraulic conductivity, on the fourth power of the radius, makes the distinction between them and the pores essential. The effects of occlusion on gw are difficult to calculate but the general features of the sieve tube system permit one to surmise that it has been made to cope with adverse hydrostatic gradients of variable but often large magnitudes. 

As in the case of the xylem conduits, the sieve tubes communicate with the cells and tissues around them. Lateral transport systems, including the pathways for the exchange of substances between them and the xylem conduits, may be visualized as lateral branches of the long distance pathways (xTU and pTU). These offshoots (ITUs) link all regions of the cross section of the root and the shoot with the xTU and pTU. 

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