Plant structures or organs fulfil specific functions, and those functions determine the structures that perform them. Among terrestrial (land) plants, the vascular and non-vascular plants (Bryophytes) evolved independently in terms of their adaptation to terrestrial life and are treated separately here
Their common elements occur in the embryonic part of the life cycle, which is their diploid multicellular phase. The embryo develops into the sporophyte, which in turn produces spores, the haploid multicellular phase, from which arises the gametophyte stage. The haploid gametophyte then produces gametes, which may fuse to form a diploid zygote and finally an embryo. This phenomenon of alternating diploid and haploid phases common to embryophytes (land plants) is referred to as the alternation of generations. One of the major differences between non vascular and vascular plants is that in the former it is the haploid gametophyte that is the more visible and longer lived stage. In the vascular plants evolution has created the diploid sporophyte as the dominant and visible phase of the life cycle. The gametophyte phase is reduced to the pollen and ovules and is entirely contained within the sporophyte
Amongst the vascular plants the structures and functions of the Pteridophyta (ferns) which reproduce seedlessly are also sufficiently different to justify separate treatment, as here (see Pteridophytes). The remainder of the vascular plant sections address the higher plants (Spermatophytes or Seed Plants, i.e. Gymnosperms and Angiosperms or flowering plants). In the higher plants the terrestrial sporophyte has evolved specialised parts. In essence they have an underground component and an upper aerial component. The lower underground part develops roots that seek water and nourishment from the soil, while the upper component or shoot moves towards the light and develops a plant stem, leaves and specialised reproductive structures (sporangia). In angiosperms, the sporangia are located in the stamen anthers (microsporangia) and ovules (megasporangia). The specialised sporangia bearing stem is the flower. In angiosperms, if the female sporangium is fertilised, it becomes the fruit, a mechanism for dispersing the seeds produced from the embryo
Thus the terrestrial sporophyte has two growth centres, the stem growing upwards while the roots grow downwards. New growth occurs at the tips (apices) of the shoot and roots where the undifferentiated cells of the meristem divide. Branching occurs to form new apical meristems. Growth of the stem is indeterminate in pattern (not pre-determined to stop at a particular point). The functions of the stem are to raise and support the leaves and reproductive organs above the level of the soil to facilitate absorption of light for photosynthesis, gas exchange, water exchange (transpiration) pollination and seed dispersal. It also serves as a conduit for water and other growth substances between the roots and overhead structures. This occurs in specialised tissues known as vascular bundles, which give the name ‘vascular plants’ to the angiosperms. The point of insertion on the stem of leaves or buds is a node, and the space between two successive node, an internode.
Also emerging from the shoot are the leaves, عزل اسطح specialised structures that carry out photosynthesis, and gas (oxygen and carbon dioxide) and water exchange. They are lined by an outer layer or epidermis, coated with a waxy waterproof protective layer, but punctuated by specialised pores, known as stomata, which regulate gas and water exchange. The leaves also possess vascular bundles, which are generally visible as veins, and their pattern is called venation. Leaves tend to have a shorter life span than the stems or branches that bear them, and when they fall, an area at the attachment zone, called an abscission zone leaves a scar on the stem.
In the angle (adaxial) between the leaf and the stem, is the axil. Here can be found buds (axillary buds), which are miniature and often dormant branches with their own apical meristem. They are عزل خزانات often covered by leave