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The root starts to swell as the new lateral root penetrates outwards towards the surface, pushing its way through cortical parenchyma cells and finally bursting out through the epidermis into the soil. As it grows it develops xylem and phloem, which become connected with the vascular tissues of the main root.


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This process is very different from that which takes place in shoots, where lateral branches forming stems or leaves originate only from the apical meristematic tissue of the shoot. Lateral roots can develop large distances away from the root tip. Just as in roots, primary growth in stems is a result of rapidly dividing cells in the apical meristems at the shoot tip. Subsequent cell elongation then leads to primary growth.

In many plants, most primary growth occurs primarily at the apical top bud, rather than axillary buds buds at locations of side branching. The influence of the apical bud on overall plant growth is known as apical dominance , which prevents the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape.

If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Secondary Growth of Woody Plants [Animation]

The process of secondary growth is controlled by the lateral meristems, and is similar in both stems and roots. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium cambium is another term for meristem. Herbaceous non-woody plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth, or wood, is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.

The details below are specific to secondary growth in stems. While the principles are similar for secondary growth in roots, the details are somewhat different. We will discuss only the details specific to stems. The vascular cambium is located between the primary xylem and primary phloem within the vascular bundle.

Plant development

Recall that xylem is located toward the interior and phloem toward the exterior of the bundle. The cells of the vascular cambium divide and form secondary xylem tracheids and vessel elements to the inside, and secondary phloem sieve elements and companion cells to the outside. The cells of the secondary xylem contain lignin , the primary component of wood, which provides hardiness and strength. In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells , which contain a waxy substance that can repel water. The phloem together with the cork cells form the bark , which protects the plant against physical damage and helps reduce water loss.

The cork cambium also produces a layer of cells known as phelloderm , which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. The combined actions of the vascular and cork cambia together result in secondary growth, or widening of the plant stem.

These structures are illustrated below:. In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.

A new layer of xylem and phloem are added each year during the growing season. The interior xylem layers eventually die and fill with resin, functioning only in structural support. The interior, nonfunctional xylem is called heartwood.

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The exterior layers of phloem eventually become crushed against the cork cambium and are broken down. Thus a mature tree contains many interior layers of older, nonfunctional xylem deep within the stem, but only a small amount of older phloem. The layers of tissues within a mature tree trunk. The activity of the vascular cambium results in annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened.

This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem shown below. An examination of the number of annual rings and their nature such as their size and cell wall thickness can reveal the age of the tree and the prevailing climatic conditions during each season.

In this study, we used the mega-phylogeny constructed under the MB backbone. Functional trait data were obtained from the TRY Kattge et al.

Description

We included only species that had all three kinds of data occurrences, phylogenetic information, and traits. We used this resolution because it is commonly used in macroecological studies of beta diversity e. The number of species per cell assemblage were obtained by summing all columns; assemblages with fewer than 10 species were removed due to their potential to produce spurious results Kreft and Jetz, The resulting PAM included a total 3, assemblages and 12, species.

These indices reflect fluxes of water related to habitat productivity and the proportional drying power of the environment Qian et al. These environmental measures are standard variables for describing climatic characteristics of species ranges and are commonly used in macroecological studies of plants Qian et al. To reduce collinearity and variability of environmental variables we used principal component analysis PCA. Prior to implementing PCA, we standardized all variables to a mean of 0 and standard deviation of 1. Beta diversity was measured for each dimension of vascular plant diversity i.

The equations used to calculate taxonomic and phylogenetic beta diversity and their components are detailed in Baselga and Leprieur et al. Briefly, phylogenetic diversity is calculated as the total branch length of a tree that involves all co-occurring species in a community, and functional beta diversity uses the same formulae using functional dendrograms for the same set of species see below. Prior to calculating functional beta diversity using the three functional traits i. We chose Euclidean distances because our functional traits are continuous data.

The resulting functional dendrogram represents a continuous measure of functional diversity FD , where high FD indicates high species differences in functional space, whereas low FD indicates that species are more similar in functional space Petchey and Gaston, FD was determined by summing the branch lengths of the functional dendrogram that connects all species co-occurring in the community Safi et al. Subsequently, functional beta diversity was calculated using the functional dendrogram.

The same equations used to calculate the phylogenetic beta diversity see Equations 2A—C in the Supplementary Material were used for functional beta diversity. Similar to the multivariate trait dendrogram, we built individual trait dendrograms and repeated all functional beta diversity calculations for each individual trait i. For example, seed mass SM influences dispersal, with small-seeded plants generally having higher dispersal capacity than large-seeded plants; however, larger seeds are typically better provisioned and may confer advantages in establishment Westoby, ; Moles et al.

Specific leaf area SLA is related to resource acquisition Reich, , photosynthetic capacity Wright et al. Finally, whole plant height WPH is associated with establishment and resistance to environmental disturbances Moles et al. A focal assemblage and its neighbors were selected using defined window sizes and beta diversity was calculated as the mean beta diversity between each focal assemblage and its neighbors see Figure S3.

All calculations were performed in R v3. Using the beta diversity calculations i. We evaluated the structure and spatial distribution of beta diversity using spatial correlograms Diniz-Filho et al. We controlled for spatial autocorrelation using Clifford's method to obtain effective degrees of freedom for Pearson's coefficients Clifford et al. We evaluated the influence of the environmental variables on beta diversity using ordinary least-square OLS models. Because regression residuals tend to display high autocorrelation in spatially structured data Diniz-Filho et al.

SAR models account for spatial autocorrelation by adding an extra term autoregressive in the form of a spatial-weight matrix that specifies the neighborhood of each assemblage and the relative weight of each neighbor Kissling and Carl, In addition, to explore the local influence of the environmental variables on beta diversity we used geographically weighted regressions GWR Fotheringham et al.

All statistical analyses were performed in R using the packages sp Pebesma and Bivand, , spdep Bivand and Piras, , spgwr Bivand et al. Interestingly, patterns of turnover and nestedness for specific functional traits Figures 2D — F were more divergent than were taxonomic, phylogenetic, and multivariate functional axes of diversity. Table 1. Spatial correlations for beta diversity and its components turnover and nestedness for the three dimensions of biodiversity taxonomic, functional, and phylogenetic.

Figure 2. Spatial patterns of beta diversity for three dimensions of North American vascular plant diversity and individual traits based on the proportion of beta diversity explained by turnover vs. These results support the hypothesis that historical processes e. Finally, we found that environmental conditions had differential associations with patterns of nestedness and turnover Table 2. These results are most evident when the local influence of the environment on beta diversity components are examined Figure 3.

Table 2. Standardized coefficients of environmental effects on beta diversity components. Figure 3. Patterns of species diversity and composition within local communities and among regions are shaped by different historical and ecological processes Ricklefs, ; Chesson, ; Cavender-Bares et al. In this study, we used large datasets of vascular plant diversity in North America and applied procedures partitioning beta diversity Baselga, ; Leprieur et al.

More specifically, we found high spatial correlation among taxonomic, functional, and phylogenetic dimensions of diversity, suggesting that these axes are coupled and may be driven by similar processes Figures 2A — C. We also found that vascular plant beta diversity varies considerably across North America, with turnover more influential in biomes with higher species richness and greater environmental stability and nestedness more influential in species-poor biomes characterized by high environmental variability Figures 2 , 3 ; Figures S1, S3.

In other words, large-scale, long-term evolutionary and environmental processes are reflected in contemporary assembly of local and regional floras across North America.