Together, xylem and phloem tissues form the vascular system of plants. Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs.
The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends.
It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill. Microscopic view of a 3-year-old pine stem Pinus showing resin ducts, rays and three years of xylem growth annual rings. In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids.
This difference in cell size and density produces the conspicuous, concentric annual rings in these woods. Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods.
See Article About Hardwoods See Specific Gravity Of Wood T he following illustrations and photos show American basswood Tilia americana , a typical ring-porous hardwood of the eastern United States: A cross section of the stem of basswood Tilia americana showing large pith, numerous rays, and three distinct annual rings.
The large spring xylem cells are vessels. In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings.
Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab. Poussart, P.
Geophysical Research Letters 3: L Anatomy Of Monocot Stems M onocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings. They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue.
Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn Zea mays : A cross section of the stem of corn Zea mays showing parenchyma tissue and scattered vascular bundles.
The large cells in the vascular bundles are vessels. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem.
In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem.
The massive trunk of this Chilean wine palm Jubaea chilensis has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems. Palm Wood T he scattered vascular bundles containing large porous vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm. Cross section of the trunk of the native California fan palm Washingtonia filifera showing scattered vascular bundles.
The large cells pores in the vascular bundles are vessels. The palm was washed down the steep canyon during the flash flood of September The fibrous strands are vascular bundles composed of lignified cells. Right: Cross section of the trunk of a California fan palm Washingtonia filifera showing scattered vascular bundles that appear like dark brown dots.
The dot pattern also shows up in the petrified Washingtonia palm left. The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood right is caused by the larva of the bizarre palm-boring beetle Dinapate wrightii shown at bottom of photo. An adult beetle is shown in the next photo. Through a specialized heating process, the natural sugar in the wood is caramelized to produce the honey color.
Vascular bundles typical of a woody monocot are clearly visible on the smooth cross section. The transverse surface of numerous lignified tracheids and fibers is actually harder than maple. Table 2.
The model calculates xylem and phloem pressure and sugar concentrations and their within tree axial gradients in steady state. Pressure differences drive xylem and phloem transport, i.
Phloem sap viscosity was made to be sugar concentration dependent. Transpiration rate, phloem loading made equal to photosynthesis rate and unloading rates and soil water potential were given as boundary conditions, and xylem and phloem hydraulic conductance and tree height were given as structural parameters. The position of phloem unloading could be varied in the transport model so that we ran simulations where phloem unloading was made to occur either evenly along the phloem transport pathway or exclusively in the roots.
We did three simulations with the model. We used a 10 m pine with a maximum sapwood depth of 2 cm as an example, and took the axial distribution of xylem and phloem conductivity from the equations shown in the Appendix A3 and demonstrated in Figure 7. In other words, the total amount of phloem tissue was preserved, but was distributed unevenly as a function of axial position.
For this, we used the scaling relations for whole tree xylem and phloem hydraulic conductance as a function of tree height derived in Appendixes A1 and A2 and demonstrated in Figure 4 and Table 4 for the case of pine.
In this simulation, leaf gas exchange rates i. Photosynthesis rate was made to be proportional to transpiration rate.
Note that this resulted in different initial values for phloem conductance between the cases where no heartwood was assumed and the case where the maximum sapwood depth was set at 2 cm. We also varied the initial value of phloem conductive and distribution of phloem unloading to see their effects on the results. The equations obtained for the scaling of xylem and phloem properties as a function of tree height L starting from a distance L 0 from leaf apex are as follows.
The derivation of the equations is presented in the Appendix A1. These equations apply only to the case without heartwood. The numerical equations for the whole tree scaling relations including sapwood to heartwood turnover are shown in Appendix A2.
The values of 0. Xylem and phloem properties were given constant values at branches than smaller than this. The values for L 0 were chosen large enough so that we had measurements from branches of corresponding diameter.
The cross-sectional area of the whole bark A b , i. The scaling exponent ranged from 1. The scaling exponents were rather close to each other across the species. When testing the difference, the logarithmic transformation changed the exponents somewhat 1. Instead, the bark was divided into outer and inner bark, and the latter represents the functional phloem tissue.
Figure 1. For inter-species comparison of inner bark thickness there was sufficient data for aspen and pine. Their exponents were not significantly different from each other in the ln-transferred data. Figure 2. Measured inner bark i. Nitrogen content increased clearly with decreasing stem diameter in both the living bark and the whole bark, but remained fairly constant for the xylem Figure 3. While all species seemed to follow similar pattern for the living bark, there seemed to be a level difference for the whole bark so that there was the most nitrogen in the aspen bark and least in pine bark for the same diameter.
Figure 3. The data measurement points in living bark and xylem were from 8 trees 4 species. Table 3. Whole tree scaling relation predictions were made for two example species: pine and aspen. The allometric relations used in the scaling of whole tree xylem and phloem volume, nitrogen content and hydraulic conductance are presented in Table 3. Figure 4 shows the scaling relations for phloem and leaf properties in relation to the xylem properties, and Figure 5 shows the absolute values for xylem, phloem and leaf properties.
Aspen had a larger amount of phloem and higher phloem to xylem ratio in relation to pine. Leaves were the largest sink of nitrogen in small trees, but xylem and phloem exceeded the leaves as a nitrogen sink with increases in tree height Figures 4C,D , 5E,F. The total nitrogen content of the phloem was smaller than that of the xylem in pine and large aspen trees. The total nitrogen content of the phloem exceeded the xylem nitrogen content in small aspen trees Figures 4C,D , 5E,F.
Assumptions on heartwood proportions and nitrogen content of the heartwood caused the relative nitrogen contents between the tissues to vary strongly. However, when there was no heartwood, or the nitrogen content of heartwood was assumed to be same as that of the sapwood, then the role of the phloem as a nitrogen sink decreased in relation to xylem with increases in tree size.
Table 4 present the absolute values for scaling of tree xylem and phloem volume, nitrogen content, conductance, and leaf area-specific conductance as a function of tree size. Note that scaling is not strictly allometric [see Equation 2 and Appendix A2], although very close to it, for each case.
Figure 4. The predictions for the whole tree phloem volume in relation to xylem sapwood volume A , phloem hydraulic conductance in relation xylem hydraulic conductance B , total phloem and leaf nitrogen content in relation to xylem hydraulic content for the scenarios in which the heartwood has the same nitrogen content as the sapwood and for the case of no heartwood C , and phloem and leaf nitrogen content in relation to xylem hydraulic content for the case where the heartwood has the same nitrogen content as the sapwood D.
In B the same area-specific conductivity was assumed for xylem and phloem. Figure 5. The predictions for the absolute values for whole tree volume of xylem and phloem A,B , hydraulic conductance of xylem and phloem C,D , nitrogen content of xylem, phloem and leaves E,F , and hydraulic conductance of xylem and phloem per leaf area G,H as a function of tree height. Table 4. The results for scaling of tree properties as a function of tree height L.
Figure 6 shows the minimum and maximum xylem and phloem volume, nitrogen content and conductance in relation to a 10 m tree obtained from the sensitivity analysis done with parameter combinations. The general trends within remained unchanged, although the xylem, phloem and leaf properties overlapped with each other. Xylem and leaf properties seemed to be more sensitive to parameter combination than those of phloem. Figure 6. The minimum and maximum xylem sapwood and phloem volume A , nitrogen content B and conductance C in relation to a 10 m tree obtained from the sensitivity analysis done with parameter combinations.
Also total leaf nitrogen content is shown in B. Within a 10 m tree taken as an example here phloem cross-section and volume was distributed very much toward the apex, whereas xylem sapwood cross-section was evenly distributed axially, following from our pipe model assumption Figure 7A. Xylem and phloem nitrogen content were more concentrated toward the apex Figure 7B , but this relation was much stronger for the phloem, especially for aspen.
Assuming maximum sapwood depth to be 2 cm caused phloem conductance to be distributed more evenly within the transport axis. The axial distribution of xylem and phloem properties was very similar in pine and aspen for cross-sectional area and conductance, but differed greatly for nitrogen content. Figure 7. The axial distribution of xylem sapwood and phloem cross-sectional area A , nitrogen content B and hydraulic conductivity in a m tree C.
Values are expressed in relation to tree base in each case. The xylem pressure water potential drop was predicted to occur more steeply close to the apex, while phloem pressure drop was predicted to occur more at the tree base in Figures 8A,B , particularly when phloem unloading occurred in the soil. Phloem pressure gradients were sensitive to heartwood assumptions. In the absence of heartwood formation, phloem hydraulic conductivity was more concentrated toward the apex see Figure 7C , which resulted in the phloem turgor pressure drop to concentrate more toward the base of the tree Figure 8B.
Phloem osmotic concentration gradient, which results from the interplay between both xylem and phloem transport properties, was predicted to be more evenly distributed over the transport axes. The normalized pressure and concentration gradients shown in the figure were not very sensitive to parameterization of the model, but the absolute values naturally were not shown.
Figure 8. Simulated xylem water potential, phloem turgor pressure and phloem osmotic concentration axial profiles for cases of phloem unloading in sink and phloem unloading along the stem for pine with an assumption of maximum sapwood depth of 2 cm A and no heartwood formation B.
Importantly, the optimal axial allocation of phloem tissue predicted by the model was never as large as in the scaling results from the measurements, i.
Finally, in simulation 3, we analyzed how the whole tree level turgor pressure difference varies as a function of tree height using the predicted structural scaling of whole tree xylem and phloem hydraulic conductance.
Phloem turgor pressure was predicted generally to increase slightly with increases in tree height when no heartwood formation was assumed, and to decrease slightly when maximum sapwood depth was limited to 2 cm Figure 9.
As the actual amount of sapwood can be predicted to lie in between these extreme scenarios, the turgor pressure differences between the leaves and roots could thus be expected to remain rather stable with increases in tree height. Phloem became unable to transport all of the assimilated sugars in trees larger than 15 m only in the case of low initial phloem conductivity and the assumption of no heartwood formation.
In this case phloem sap viscosity experienced a sharp build up preventing an increase in the phloem transport despite an increase in the turgor pressure gradient. The increase in turgor pressure difference with increasing tree size was more pronounced when sugar unloading occurred exclusively in the roots Figure 9A in comparison to phloem unloading occurring evenly along the stem Figure 9B.
In many of the cases presented, phloem turgor pressure difference increased with increasing tree height for small trees, but then started to decline again. This was due to gravity which started become important for taller tree. Gravity aids phloem transport while decreasing the capacity of the xylem to transport water to the leaves. According to the isohydric scenario presented here, the decrease in xylem transport led to lower leaf exchange rates and thus also for a smaller transport need for the phloem.
Increase in the initial value for phloem conductivity decreased the turgor pressure gradient for all tree sizes, as would be expected. Importantly, the turgor pressure difference between the leaves and roots required to drive the phloem transport of the assimilated sugars was predicted not to increase linearly with increases in tree height.
Figure 9. Simulated turgor pressure difference between leaf and root phloem as a function of tree height with varying parameterization for a case where all phloem unloading occurs in the root A and evenly along the stem B for pine. Pine was used as an example species in all of the simulations done with the xylem and phloem transport model, but the corresponding simulations for at least aspen would yield similar results as the scaling relations for the xylem and phloem volumes and hydraulic conductances are quite similar amongst the species see Figure 4 and Table 4.
The equations constructed in this study make it possible to estimate whole tree level xylem and phloem properties volume, hydraulic conductivity, nitrogen content.
Predictions can be made on how whole tree level properties scale with tree size assuming that the measured relationships do not change with tree height. This was supported by the data presented here on trees that varied in size measured for four different species. The approach presented here can also be connected to functional-structural tree models that often provide detailed description of tree axes and their dimensions e.
Phloem volume and nitrogen content were predicted to be concentrated heavily toward the tree apex, in contrast to the xylem, whose properties were more evenly distributed within a tree Figure 7.
Partially the latter was due to the pipe model assumption for the xylem. However, the pipe model assumption has been shown to work quite well for all the species analyzed in the measurements Kaufmann and Troendle, ; Ilomaki et al. Also phloem transport capacity hydraulic conductance was concentrated more toward the apex, especially if heartwood formation was limited.
In contrast, xylem conductance was concentrated toward the base. In both cases the translocation capacity is thus largest closest to the source of the principal transported substance.
For example, if nitrogen content sampling was done exclusively from larger stem and branch parts, then the total amount of nitrogen allocated to the vascular tissues would be grossly underestimated. This result has also direct implications to forest management where bioenergy harvesting is becoming more popular with the need for boosting the use of renewable energy sources.
Our results imply that removal of distal parts of the crown from the growing site will deplete the ecosystem nitrogen pool as efficiently as the removal of leaves. The relations between xylem sapwood and phloem volumes and conductances at the whole tree level were found to be sensitive to the assumption made about sapwood turnover to heartwood. When no heartwood formation was assumed, whole phloem conductance could not keep up with xylem conductance with increase in tree height.
However, when a maximum sapwood radius of 2 cm was assumed, whole tree xylem and phloem conductances were predicted to change at approximately the same rates with tree growth, and xylem sapwood to phloem ratio was predicted to saturate approximately to a value of 10 Figure 4A. It seems clear that the xylem and phloem become increasingly larger sinks of nitrogen in relation to foliage with increases in tree height, and that the nitrogen requirements of the vascular tissues could be a major limiting factor to tree growth in the Boreal region.
This means, for example, that sucrose is transported:. Applied chemicals, such as pesticides , also move through the plant by translocation. Xylem and phloem Plants have tissues to transport water, nutrients and minerals. Xylem and phloem in the centre of the plant root This table explains what is transported by the xylem and phloem: Tissue What is moved Process Xylem Water and minerals Transpiration stream Phloem Sucrose and amino acids Translocation Xylem Mature xylem consists of elongated dead cells, arranged end to end to form continuous vessels tubes.
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