Friday, December 29, 2006

Xylem tracheids and trachea - conducting tissue or not?

The raw materials needed for metabolism in a plant should come from its surroundings. The nutritional demands vary from time to time and phase to phase in a plant. Due to practical limitations small plants or cuttings are used for experimentation. Quantitative and qualitative measurements may be made in relation to the growth of the plant body by controlling external conditions like climate, substratum, air, water etc. The metabolic process within the plant body may be measured indirectly by biochemical or biophysical methods (For example, quantifying the oxygen consumed or carbon dioxide released, increase in volume, shape, density, weight, number of cells, their arrangement etc.) The energy consumed or released may be calculated to an approximation using some or all parameters, usually with the help of biostatistical methods.

In spite of the availability of voluminous literature, so far there is no satisfactory explanation regarding the medium or modus operandi of conduction in plants. Few speculative hypotheses have been put forward but no conclusive proof is given (Bollard 1960, Nobel 1974, Peel 1974). But why? Theoretical or conceptual errors might have given rise to faulty techniques, faulty techniques might, in turn, have contributed to the limited points of view; limited information hinders the total vision.

Theoretically the plant body has to be considered as an organism in a continuous vital process. Due to structural and functional differences, unlike animals, in plants there is no question of "maturity" at any level of organization. What is observed is a process of ontogenetic differentiation, specialization and in some cases degeneration. Unlike animals, plants do not take in food nor possess various structural and functional systems. The absence of such "systems" in plants implies a physiological difference. (See next chapter).

In an organism with millions of cells individual cells should not be treated as separate entities. Only for the sake of understanding is this distinction made at this point. When the plant body commences either in the form of a zygote (sporophyte) or a spore (gametophyte) there is an initial stage of naked protoplast which secretes a cell wall of its own all around. At each successive division the protoplast is partially separated by the formation of the cell wall only in one facet. However, plasmodesmata maintain the continuity of the protoplasm. In other words, in any given cell all the facets are not necessarily produced at the same time and the walls do not separate the protoplasts totally. Structurally similar cells may exhibit physiological differences; it is obvious that all vital processes occur only in the protoplast. Plant cells are classified as living or dead on the basis of the presence or absence of protoplasts within them. When the protoplast is absorbed or dissolved, the cell wall remains and is considered to be a dead cell for all practical purposes. Living and dead cells may coexist in a plant, whereas in an animal the dead cells are constantly removed from its body. Also, in animals the tissue demarcation is evident due to structural differences between the constituent cells. The absence of plasmodesmata in animals is accompanied by the absence of cell walls. Therefore, theoretically there is no structural comparison between plant and animal bodies; it should be so functionally as well.

In plants there is a synthetic process in the reception, conversion and accumulation of electromagnetic energy into chemical energy after enjoying the "vital energy" (bioenergy) during the metabolism. Whereas in animals, this accumulated chemical energy is released and partly utilized during vital biochemical reactions.

When the plant body is perceived in this fashion as a continuum, the perspective changes. The ontogeny should be taken into consideration as a whole, both in space and time, and not as a few instances here and there in this process. Partial views do not yield a complete picture. Conduction in plants has been studied but with faulty techniques. The methodology used to obtain evidence in support of the hypotheses proposed so far on the xylem transport warrants some comments. Two methods are described and are generally employed to obtain 'xylem sap' for analysis. In both cases assumptions and presumptions contribute to the factual errors.

1. "Xylem" is taken for granted as a conducting tissue and hence the exudate is taken from it to study conduction.

2. Little or no attention has been given to the structural complexity involving different cell types that make up the "xylem", although elsewhere in plant anatomy elaborate descriptions are made regarding the protoxylem, metaxylem and secondary xylem. They differ in their origin, cytology, cell types and contents.

3. No effort is made to identify the element from which the "xylem sap: is obtained. In the literature when xylem vessel is considered as a conducting element no reference is made as to whether or not the adjacent xylem parenchyma is involved in the process. In what way does it differ if the xylem is devoid of vessels?

4. Structural continuity of the xylem is not considered with reference to the entire plant. In other words, the three dimensional distribution of xylem parenchyma, xylem vessel members, xylem fibers and xylem tracheids in the protoxylem, metaxylem and secondary xylem is generally ignored.

5. In one and the same vessel the constituent cell-initials near the meristems contain protoplasts, whereas farther away mere cell walls remain.

6.What is observed in laboratory plant materials (limited in quantity and quality) is presumed to occur in nature.

One of the techniques called "root bleeding" obtains exudates from the "cut ends nearest to the base of the stem." In anatomical literature this region corresponds to the hypocotylary area in most of the plants studied. In taxa where the cotyledons remain embedded within the seed coat during germination, the first internode corresponds to this region. In monocotyledonous taxa this may be the cotyledonary node. This is the so called "transition region" where there is "splitting, twisting, torsion, fusion, reorientation etc." of the "vascular bundles" or there is anastomosis of "conducting elements." In any case, it is impossible to isolate "xylem." This situation may be further complicated in diotyledonous taxa with the onset of activity in the "vascular cambium" to produce "secondary xylem" not completely distinguishable from the metaxylem. In the case of clonal multiplication, as is frequently done in horticulture, the adventitious roots arise from the callus produced from the dedifferentiated parenchymatous cells of the shoot. At this region, as in any other part of the plant, the "xylem" consists of cells with protoplasts and those devoid of the same, usually intermixed. Therefore, attempts to obtain "root exudates from cut ends nearest to the base of the stem" requires refined techniques which can verify the source of the exudate with precision. In science where accuracy and precision are basic principles such techniques are examples of how they should not be!

The second technique used obtains xylem exudates from the sap wood. "Recently matured xylem elements" sounds attractive but is not precise. First of all, to locate the "mature" elements and to sharply circumscribe the cell(s) involved will be a perpetual problem when the plant body is perceived as a continuum; dynamic vitality is the secret of totipotency. After division, differentiation, specialization and in some cases degeneration a cell derived from the meristematic region becomes "mature." While an adjacent cell is still at the division process closer to the meristematic region, the derived cell farther away might exhibit all or some of the stages of differentiation, specialization or degeneration of the protoplast. Only by the deposition of the secondary wall material with the absorption or degeneration of the protoplasts the tracheary cells attain their "maturity." Again, it is to be noted that in xylem the cells adjacent to a vessel may be parenchyma, tracheid or even another vessel member, not necessarily at the same stage of maturity. Whatever be the case, there is no horizontal perforation plate between the vessel member and the adjacent cell(s). When three dimensional growth process in plants is conceived as such, the longitudinal as well as the horizontal protoplasmic continuity will have to be taken into consideration. The intercalating cell walls (cellulosic ones deposited secondarily after the elongation and adjustment of the cells) may partially interrupt the protoplasmic continuity. If a cell has attained physiological maturity all activities related to the metabolism should be located in that particular cell. This implies that a lot of the biochemical and enzymatic processes can take place only if the protoplast is intact. In other words, symplastic activity carries out these vital processes, whereas the apoplastic parts of the plant cannot participate actively in such processes. Both climatic and edaphic conditions play an important role in the process of plant growth. The phenophase at which each plant is found at the time of obtaining the samples of the "xylem sap" is also of importance. In the tropics, where there is apparently no such seasonal changes as in temperate regions, plants of the same taxon may exhibit different phenophases, sometimes even in one and the same individual. The "xylem sap" constitutes part of living protoplasm since it contains metabolic products like amino-acids, carbohydrates, enzyme systems, growth regulators etc. (Peel, 1974). Water-conducting vessels or tracheids, devoid of protoplast when "mature", at best should contain water and dissolved salts (raw materials) but no finished products. In the literature no mechanism is explained how the sap from the symplast gets converted into water and dissolved salts when it enters the apoplast.

Thus, both techniques used which obtained the exudate or sap either from the cut ends of roots nearest to the base of the stem or from the recently matured xylem elements of the sap wood, present theoretical and practical drawbacks.

The aphid technique to study the phloem exudate depends on the selectivity of these insects. It is assumed that the phloem is the conducting tissue. The protoplasmic ultra structure involving endoplasmic reticulum, organelles, nuclear material and other cytoplasmic inclusions are aspects not taken into consideration to explain the functional details of the tissue.

Evert (1982) says "The tracheary elements of the xylem have rigid walls and lack protoplasts at maturity. These structural characteristics facilitate the rapid movement of water and dissolved solutes under considerable tension induced largely by transpirational loss of water from the surface of the leaves." There is no proof in the literature for this statement. Stem cuttings devoid of leaves do not transpire but there is absorption and conduction within. It is interesting, however, that the same author, with reference to phloem, considers "In order to have a clear understanding of translocation it is essential to correlate the structure with the function of the tissues and cell types concerned." If this is true for phloem, criteria should not vary when xylem is under consideration.

Another aspect of the language used in descriptive anatomy is worth considering here. Although these statements are common to any text book on general botany or plant anatomy for the sake of clarity the following sentences are quoted from Fahn (1982):

Page 188: ".....the primary vascular cylinder is interrupted at each node by the exit of one or more bundles that enter the leaves."

Page 189: " is necessary to study more accurately the nature of the traces and to follow their passage downwards in the stem."

Page 190: ".....Leaf trace bundles was used to designate those that directly connect the leaf and stele. Cauline bundle refers to those bundles that form the major vascular system of the stem and which may anastomose and give rise to leaf traces. The term common bundle has been used for those bundles that run unbranched for a relatively long distance in the stem and which eventually terminate in a leaf trace." ".....the vascular system of the stem can be interpreted as being a system of leaf traces that continue downwards in the stem for one or more internodes where they fuse with the leaf traces of the lower nodes."

Page 193: ".....These penetrate deep into the interior of the stem and then pass to the periphery in lower nodes."

A reader of such statements interprets this as if here is a physical movement or material motion. No effort is being made to explain how, during, ontogeny, differentiation and specialization as a process produce such configuration of cells or groups of cells in the plant body, or how the final external or internal structure appears to be a network formed of cells or groups of cells in a specific manner as a consequence of morphogenetic expression. "Dichotomously branched veins" commonly found in ferns again refers to the final expression and has nothing to do with the dichotomy produced by the splitting of the apical meristem into two equal halves each developing into a branch. Whether it is a conceptual error or the limitation of the language is the question. Such ambiguous interpretations should be avoided in science. Other terms equally ambiguous are: "conducting strand." "vascular tissue," "higher plants or lower plants." These are products of speculation or anthropomorphism (see Font Quer 1973 for other terms). The description of the "transition zone," as exhibiting "splitting, twisting, torsion, fusion, etc. of bundles" is misleading. First of all the roots have no such bundles, because the xylem and phloem elements are not found in the same radius, secondly the individual cells constituting the xylem and phloem units do not suffer such torture during differentiation, specialization or degeneration.

It is again impossible to detect vascular bundles down below the stem, especially in trees or other perennials where the onset of the secondary growth is very close to the apical meristem, sometimes only a few plastochrons below. Such being the case, tracing the individual bundles several nodes below is next to impossible. The nodal plate formation, especially in monocotyledonous taxa, is a typical example to show that the bundles do not "run" beyond one internode. In plants devoid of leaves the internodes present xylem and phloem in distinct patterns as bundles where it is again impossible to talk about leaf traces. The presence of intercalary meristems at the nodal region, as in many monocotylonous taxa, or at the base of the primordial of lateral appendage or at the base of the fruit stalk or even the fruit itself is a significant feature. There are meristematic cells or in other words, cells that have the capacity to undergo division and differentiation interrupt the continuity of xylem and/or phloem strands. The network of xylem and phloem elements within the leaf blade is a product of differentiation and specialization in situ. The final form may look like a profusely branching network. Even then the direction of vascular differentiation is described as basipetal. That is to say that the xylem or phloem elements get "mature" at the leaf tip and then progressively downward. Hence, the last region where xylem elements get differentiated will be the leaf base corresponding to the intercalary meristem. The so-called leaf trace within the leaf differentiates basipetally but within the stem acropetally. In other words, the meristematic region gets differentiated finally. The continuity of the xylem or phloem is interrupted accordingly.

In tropical trees it is common to observe the intact leaves on stems and branches that have undergone secondary thickening. In these cases the vascular cambium has produced a sufficient amount of secondary xylem and secondary phloem between the primary phloem of the stem, thus occupying the region of leaf gap. In other words, once the cambial activity is evident the xylem of the stem does not maintain organic connections with the primary xylem of the leaves. The cambial activity is so extensive that the primary phloem of the stem is either obliterated or even thrown away as "periderm" following the activity of the cork cambium. How then is it possible to maintain the water supply through the xylem? If symplastic and apoplastic paths could be combined, as has been done in the recent literature, special mechanisms will have to be proposed for transfer of water and dissolved salts repeatedly from the symplast to the apoplast and vice versa.

Other structural considerations serve to clarify the process. For example:

1. Xylem parenchyma cells occur in between the protoxylem vessel elements and the metaxylem vessel elements (may be any other tracheary elements). Within the desmogen those elements which differentiate and specialize with secondary wall thickenings are optically distinguishable from the neighbouring elements devoid of such peculiarities.

2. Protoxylem element (irrespective of the type of element) gets obliterated either by stretching of the primary body or by the rupture of the secondary wall material as a consequence of the stretching. The protoplast is already used up during the cell- ontogeny. In monocotyledonous taxa, where these elements were present, empty spaces (lacunae) occur at maturity. These cells or groups of cells along the same location nearer the meristem may have protoplasts and hence physiological activities are possible in this region.

3. Away from the meristem, on the other hand, when there is additional differentiation and specialization, metaxylem elements are formed out of the desmogen. These metaxylem elements are not in longitudinal continuity with the protoxylem elements, nor are they found adjacent to one another. At least the vessel elements are separated by other types of cells in between.

4. When the fascicular cambium is present (the undifferentiated cells of the demsogen in between the differentiated ones), the production of "secondary" elements out of these fusiform initials is in the proximity of the metaxylem. The gradation is smooth that one can hardly distinguish between the late formed metaxylem element and the early formed secondary xylem element. Moreover, the differentiated and specialized cells in either case may be tracheids, fibers, vessel members or parenchyma cells at `maturity.'

5. In monocotyledonous taxa where the primary growth is maintained throughout, a bundle sheath formed of sclerenchymatous cells is common; this is best seen in grasses. In order to explain the conduction through the xylem or, for that matter, phloem elements, it is necessary to show how water and the dissolved solutes traverse through the dead cells (sclerenchymatous ones), longitudinally and transversely. In such case even the sclerenchymatous bundle sheath should have been called "conducting tissue."

6. The absence of a distinct arrangement of bundles or even the separation of xylem and phloem elements at the nodal region is quite interesting compared to the clear-cut bundle appearance at the inter nodal region. The anastomoses of these elements at each node under primary growth at the apical regions of the shoot should be noted.

7. The interpolation of newly differentiated and specialized elements derived from lateral meristem in the axis of the plants (in dicotyledonous and gymnospermous taxa) between the primary xylem and primary phloem, maintains the discontinuity.

8. The early onset of cambial activity produces significant quantities of secondary xylem and phloem elements in between the primary elements of the lateral appendages of limited growth and the primary body of the stem axis. This separation of primary elements, even if the individual cell continuity is overlooked for the time being, is of great importance at the time of consideration of these elements as conducting in function. The elements of the primary phloem may get differentiated to "cortical fibers" when the cortex is intact or it may be sloughed off as periderm after the activity of the phellogen. At the same time the primary phloem elements in the appendage are still alive and are actively participating in the physiological functions of the protoplast.

9. The presence of tracheary elements and phloem elements in the mesophyll of the leaf blade is interpreted as an evidence of conduction. However, it is interesting to note that such arrangements are not seen in cladodes where the cortical cells are replete with chloroplasts. Does it mean that when there are cladodes no conduction is required to supply the soil solution to the place of synthesis?

10. Cut flowers or stem cuttings stay fresh in water in spite of the absence of absorbing root ends.

11. At the embryonic level there is xylem and phloem differentiation but the water and soil solution is not absorbed nor conducted from the surroundings through the `vascular strands.' The energy required is available either from the surrounding endosperm or is stored in the cotyledon(s). The xylem and phloem elements in the embryo do not have organic connection with the xylem and phloem of the plant bearing the seed or fruit.

12. In all aquatic plants (whether attached to the substrate, suspended or free-floating), though to a lesser extent, there is differentiation of xylem and phloem elements.

13. In grafts the success depends on the callus formation and the establishment of organic continuity between the protoplasts rather than the connection between the xylem or phloem elements.

14. Colouring substances like safranin normally used to optically distinguish the tracheary elements of the xylem have special chemical reactions with the wall material deposited. Any experiment wherein the walls get coloured does not "prove" that the conduction is through those cells but only signifies that the colouring matter has reached that region of the organism, and consequently reacts with the wall material making the wall optically distinguishable from the neighbouring cells where such reaction has not taken place. The colour is not manifest in the protoplast even when safranin or other colouring substances pass through the same. This only proves that the chemical reaction has not taken place or the intensity of the colour is too little for observation or that the concentration of the colouring matter is too dilute. With these points under consideration the following conclusions have been drawn:

1. The absorption of water and solutes dissolved in it or in air occurs through the living cells at the surface of the organism.

2. Once within the protoplast, the raw materials serve in the metabolism of the organisms.

3. When the supply of raw materials is abundant the protoplast increases in size, improves in quality - within the limits of specific range and may divide partially to maintain more physiologically active units, the cells. Each derived cell is a continuation of the previous one through plasmodesmata. In other words, the wall formation along only one facet at a time is incomplete because of the presence of pits.

4. Each living cell (meristematic, parenchymatous, epidermal, companion cells, ray cells, phellodermal cells, lenticels etc.) utilize whatever raw material it needs, for the specific physiological activity it has to perform, from the absorbed solution. The raw materials not used and the metabolic products formed within the protoplast pass on to the adjacent cells through plasmodesmata.

The limited number of mutilated and degenerated protoxylem elements are incapable of transporting enormous quantity of water and solvents absorbed by the root hairs.

5. All such movements are possible due to the "vital force" or the "bioenergy" within the protoplast.

6. Tracheids, vessel members or vascular elements and sclerenchymatous cells are formed by the deposition of metabolic products or waste material of the organism.

7. These cell walls, frames devoid of any protoplast, compactly arranged at regular patterns provide the organism with necessary mechanical strength and also serve as reservoirs of waste materials. They do not participate in any live processes including conduction.

Literature cited

1. Bollard, E. G. 1960, Transport in xylem, Ann. Rev. Plant Physiol. 11: 141-166.
2. Esau, K. 1977, Anatomy of Seed Plants, 2nd Ed. John Wiley and Sons, New York.
3. Evert, R. F. 1982, Sieve-tube structure in relation to function. Bioscience 32 32 (10); 789-795.
4. Fahn, A. 1982, Plant Anatomy, 3rd Ed., Pergamon Press, Oxford.
5. Font Quer, P. 1979, Diccionario de Botanica. Editorial Labor, Barcelona, Spain.
6. Grew, N. 1682 The Anatomy of Plants, Johnson Reprint Corporation, New York (1965).
7. Metcalfe, C. R. and L. Chalk 1979 Anatomy of Dicotyledons. 2nd Ed. Oxford University Press, Oxford.
8. Nobel, P. S. 1974, Introduction to Biophysical Plant Physiology, W. H. Freeman and Company, San Francisco.
9. Peel, A. J. 1974 Transport of Nutrients in Plants. John Wiley and Sons, New York.
10. Zimmermann, M. H. and J. A. Milburn (Eds.) 1975 Transport in plants I. Pholem transport. Encyclopedia of Plant Physiology, New Series, Vol. 1. Springer-Verlag, Berlin.

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