It is the complex tissue, which acts as a transport system for soluble organic compounds within vascular plants. This emptying of the SE is essential to allow the unimpeded flow of water, signal proteins, mRNA, and photoassimilates which travel through the SE. These fibers are not, however, understood as part of either phloem or xylem; although they are of vascular nature, they differentiate directly from procambium. Strasburger cells in the secondary phloem can be either axial parenchyma cells, as is common in Ephedra [13], or ray parenchyma cells, as is common in the conifers (Figure 3c) [14]. Lists depicting these variations in all phloem cell types are of ultimate importance for complete bark descriptions [5]. Ray composition is appreciated in radial sections. It mediates most of the interactions between a plant and its environment. The sieve tube member is a mature phloem cell involved with long distance movement of food material. Rays in the secondary phloem. (f) Secondary phloem of Carya cordiformis (Juglandaceae) showing a phloem formed by a background of fibers where solitary to multiple of two sieve tubes are scattered, with sieve-tube-centric and diffuse-in-aggregate axial parenchyma. B. Upright ray cells: long axis of the cell vertically elongated (3). Secondary phloem (sp) beginning to be produced. However, because the high protein content is not always present, the name Strasburger cell, paying tribute to its discoverer Erns Strasburger, is recommended over albuminous cells [5, 12]. The primary phloem is simpler than the secondary phloem and is basically formed by sieve elements and parenchyma cells (Figure 1a–d). Phloem is the vascular tissue responsible for the transport of sugars from source tissues (ex. The total amount of xylem tissue is more. It is the complex tissue, which acts as a transport system for soluble organic compounds within vascular plants. Sieve cells lack P-protein in all stages of development. (b) Longitudinal tangential section (LT) of Cordia caffra (Boraginaceae) showing sieve tube element (se), companion cells (arrow), multiseriate ray (r), and axial parenchyma (p). Companion cells are parenchyma cells that function to load and unload material into the sieve tube member. Over-expressing APL inhibits the formation of xylem. In many species it is functional for just one growth season, while for other species they can be functional a couple of years, or in the case of plants that lack secondary growth, they will be living for the entire plant life spam. Xylem and phloem are closely organized in plants. Companion Cells. The rays in the conducting phloem have typically the same organization in terms of width, height, and cellular composition as the secondary xylem. These are the cells that control the cells’ metabolism, and they are linked together with large numbers of plasmodesmata. One or more companion cells may be associated with a single sieve element. Phloem is composed of four kinds of cells: sieve elements, companion cells,phloem parenchyma and phloem fibres. Describe the four kinds of phloem tissues. In the protostele, the entire center of the organ is composed of vascular tissue (Figure 1a), with the phloem in strands alternated with a central xylem in the protostele, haplostele, and actinostele (Figure 1a), while primary phloem is interspersed in the protostele plectostele [6]. The position of the phloem is typically external or abaxial to the xylem, but in some lineages the bundles are bicollateral (Figure 1b), and phloem is present both inside and outside (abaxial and adaxial), while in amphivasal bundles, the xylem encircles the phloem (Figure 5d), as in the secondary vascular tissues of some Asparagales [32, 33] and Iridaceae corms [34]. 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