<P> Phloem is a specialised tissue for food transport in higher plants . Phloem cells mainly transport sucrose along pressure gradients generated by osmosis . This phenomenon is called translocation . Phloem consists of two cell types, the sieve tubes and the intimately associated companion cells . The sieve tube elements lack nuclei and ribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells . Sieve tubes are joined end - to - end with perforate end - plates between known as sieve plates, which allow transport of photosynthate between the sieve elements . The companion cells, connected to the sieve tubes via plasmodesmata, are responsible for loading the phloem with sugars . The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous function known as the leptome . </P> <P> Plant epidermal cells are specialised parenchyma cells covering the external surfaces of leaves, stems and roots . The epidermal cells of aerial organs arise from the superficial layer of cells known as the tunica (L1 and L2 layers) that covers the plant shoot apex, whereas the cortex and vascular tissues arise from innermost layer of the shoot apex known as the corpus (L3 layer). The epidermis of roots originates from the layer of cells immediately beneath the root cap . </P> <P> The epidermis of all aerial organs, but not roots, is covered with a cuticle made of the polyester cutin and / or the hydrocarbon polymer cutan with a superficial layer of epicuticular waxes . The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin . Several cell types may be present in the epidermis . Notable among these are the stomatal guard cells, glandular and clothing hairs or trichomes, and the root hairs of primary roots . In the shoot epidermis of most plants, only the guard cells have chloroplasts . Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis . </P> <Ul> <Li> Cell membrane </Li> <Li> Cell wall </Li> <Li> Nuclear membrane </Li> <Li> Vacuole </Li> <Li> Plastid </Li> <Li> Chloroplast </Li> <Li> Leucoplast </Li> <Li> Chromoplast </Li> <Li> Golgi Bodies </Li> <Li> Cytoplasm </Li> <Li> Nucleus </Li> <Li> Chromatin </Li> <Li> Cytoskeleton </Li> <Li> Nucleolus </Li> <Li> Mitochondrion </Li> </Ul>

Why do animal and plant cells have a nucleus