Transport systems       




Xylem and phloem structure and function


 The structure of the xylem and phloem in higher plants has been reviewed in a number of excellent texts and we do not propose to undertake more than to highlight those aspects which we consider relevant to the accompanying exercises. We therefore advise readers to refer to any of the many excellent texts that exist in the literature.

Clearly the evolution of the conducting system, together with the development of the lignin synthesis pathway, must be amongst the important factors which contributed to the evolution of vascular plants.

Physiologically, the elements within the xylem and phloem can act independently of one another, yet the phloem relies on water provided by the xylem in order to provide the driving force required for long-distance translocation of assimilated material. Structurally, many of the elements within the xylem (with the exception of parenchymatous elements) are dead at maturity and have a highly modified wall structure. In contrast, the phloem (with the possible exception of sclerenchymatous elements) consists of cells which contain protoplasts at maturity. Sieve elements, including the more primitive sieve cells which occur in gymnosperms are unique, in that they either lack nuclei or contain only remnants of nuclei, which are of unknown functional or regulatory capacity.

Transport through the xylem is driven in part by root pressure and by the evapotranspirative processes which take place mostly through stomata, lenticels and cracks in cuticular layers. Transport in the phloem on the other hand, relies on a build-up of solutes (loaded into the sieve tubes at the sources) and the subsequent attraction of solvent to this area. Increasing pressure and the resultant enhancing of flow within the sieve tubes but away from the source, to some local or distant regions of the plant, (termed sinks) where the solutes are unloaded and utilized, is termed translocation.

In the primary plant body, xylem and phloem generally occur either in vascular bundles, (in leaves and stems) or in strands, with xylem and phloem on alternating radii in roots. In plants that have undergone secondary thickening, the xylem and phloem in roots and stems become spatially separated, but interconnected through a series of secondary rays, which are usually parenchymatous. Whilst the interrelationships are easier to follow in primary vascular tissues, clearly the development of radially-arranged ray tissues are of paramount importance in the regulation of solute and solvent transport. One could ask why doe these disparate systems occur in close proximity. Clearly, the xylem does not require any direct inputs from the phloem, but, does the phloem require or obtain any input from the xylem? Examination of the leaf blade bundles in gymnosperms and angiosperms, demonstrates close spatial relationships between the tissues. Functional sieve tubes may occur adjacent to tracheary elements, in many monocotyledonous plants, particularly amongst the grasses and sedges. These sieve tubes are the last to differentiate and mature, and curiously, they have thick walls, which in some instances (barley and wheat) have been reported to undergo lignification. Even more curious, is the lack of the identifiable companion cell-sieve tube complex, found in the early metaphloem in these plants, and commonly in all other vascular bundles in angiosperms. 




Fig. 1 A, fibre, B, tracheid and , C, vessel Intermediate cell types exist between each.

Fig. 2  A range of vessel element perforation plates and wall pitting all x 220. A, Camellia  scalariform   "scalariform" . B,  Liriodendron tulipfera, scalariform; pits opposite. Sambucus nigra, simple plate, pits alternate,  D, Euphorbia splendens, simple plate,  pits alternate. E, Scirpodendron chaeri, scalariform plate, pits opposite (from primary xylem).

Fig. 3. Xylem conduits. The images to the right show xylem elements in longitudinal section in Coleus stem. This is a dicotyledonous species. Notice the change in structure of the secondary wall thickening from left to right, with spring-like thickening, progressing to tightly-coiled  bands in the last element to the right. In contrast, the cross section of pine wood (far right) shows remarkable uniformity  in the diameter and wall thickness of these elements. 






































































Phloem anatomy is determined by process physiology



The answers to the spatial proximity of the xylem to the phloem may lie in the physiological requirements for successful phloem loading at the source, the maintenance of long-distance transport and the unloading process in local and distant sinks elsewhere in the plant.  Research carried out using dicotyledonous species have shown that sieve tubes within the same plant, vary in size, according to their position (in root, shoot or leaf). Figure 2 below illustrate the difference in size between loading, transport and unloading phloem sieve tubes and companion cells in Nymphoides. In the roots of dicotyledons, the mature metaphloem is about  5-10 µm in cross section, and the companion cells are 15 to 40 µm in cross section; the companion cell’s size increase reflecting its role in the phloem unloading process. In the leaf, sieve tubes are very narrow in diameter, their surrounding parenchymatous elements, including companion cells


Fig. 2 Micrographs showing the size change relationships between the companion cell and the sieve tubes in loading, transport and unloading phloem in Nymphoides. Left; minor vein in leaf lamina, Middle; phloem in central vascular bundle in the submerged petiole (STEM), Right; phloem strand from a root. Scale bars:  = 10 µm.































The phloem consists of a series of conducting elements associated with vascular parenchyma elements. In angiosperms, the conducting elements are referred to as sieve tube elements or sieve tube members, and these are almost always associated with specialized parenchyma cells called companion cells. Companion cells are ontogenetically related to the sieve tube members. Amongst the gymnosperms, the phloem is composed of less specialized conducting cells, called sieve cells, and these are associated with albuminous cells. The image to the left is a detail of part of the external phloem from a vascular bundle in the stem of Cucurbita pepo. The large-diameter cells which dominate this image show several sieve tubes in cross-section.  The smaller cells abutting the sieve tubes are companion cells.
















   Composition of phloem  In Dicotyledons    



Phloem tissue in dicotyledons is composed of assimilate transport cells, called sieve tube members, and their associated companion cells. Companion cells are described as being parenchymatic, but, the origin (ontogeny) is from the same mother cell that divides to form the sieve tube member. The companion cells in angiosperms are important, as they serve as ‘traffic control centers’ and regulate movement of all substances that either enter or leave the companion cell coming from, or going to, the sieve tube members. Given that mature sieve tube members are enucleate, (that is, they lack a functional nucleus at maturity) all the small proteins that are required on an on-going basis by the sieve tube members, must be synthesized in the sieve tube member under the control of the nucleus in the companion cell. This is a tricky business, and one could liken the sieve tube-companion cell complex to a comatose patient and an hyperactive nurse. The companion cell provides all essential control proteins, and takes care of the complex biochemistry that is involved in the loading transport and unloading process that occur in the sieve tube members. It also takes care of membrane maintenance in its associated sieve tube members.

 In dicots and monocots, the sieve tube members are joined end to end by their common cross walls. In this way, the cells form long sieve tubes, through which carbohydrates, as well as a host of other materials including viruses, can move. These cross walls are perforate, and allow rapid transport of materials through the sieve tubes.


   Composition of phloem  In Monocotyledons     



Monocotyledonous phloem is very similar to that in the dicotyledonous plants, in that it is composed of sieve tube members, and associated companion cells. We assume that the companion cell has a similar function in the monocotyledons, to that in the dicotyledons. Like the dicots, monocot phloem also contains parenchymatous elements, as well as fibres. The course of vascular systems in monocotyledon  has been studied for many years and is under active investigation at present. Modern microscopy techniques, including fluorescence and confocal microscopy, and stack frame imaging of whole and sectioned material have enabled researchers to understand for the first time the true complexity of many stems, including palms and Pandanaceae. With newer and more powerful techniques becoming available, a new area of comparative anatomy is emerging the study of whole vascular systems. The results of this study might well show basic types which underlie the major phylogenetic divisions in the plant kingdom.

Within the vascular bundles of monocots, the primary transport system is composed of an axial system only. Rays are a feature of secondary development, and are associated with gymnospermous and dicotyledonous plants only.


   Composition of phloem In Gymnosperms    



The phloem in gymnosperms is considered to be less specialized, and the functional phloem  consists of  well developed sieve cells, with sieve areas (no sieve plates, as in dicots and monocots) and their associated albuminous cells. In angiosperms the albuminous cells are replaced by companion cell companion cells. It is thought by those who study phloem that an evolutionary sequence can be observed, from systems in which companion cells are poorly defined and the sieve tube phloem, sieve tube elements communicate by rather scattered sieve areas on oblique walls to the most advanced in which sieve plate are very well defined and constitute the transverse end wall between elements in a sieve tube, and in which the companion cells are very well developed. Since the advanced sieve tube member has no nucleus, the organization of the element is carried out by the nucleate companion cell adjacent to it. Damage to the companion cell in this system may bring about failure of the element which it directs.