Why Stems of older trees are different from that of younger trees.
Answers
It is based on a simple human principle
i.e. Experience matters
Explanation:
In the section Ecological and evolutionary classification, it is pointed out that land plants are descended from aquatic plants. The early aquatic plants required few modifications for structural support or water and nutrient absorption, since the surrounding water fulfilled their needs. The water, far denser than the air, buoyed the plant body; the thin integument permitted a free exchange of nutrients across the entire relatively small body surface and a passive mechanism for spreading their gametes. Once primitive plants began to invade the land, however, modifications for support, nutrient and water absorption, turgidity, and reproduction were required to compensate for the absence of an aqueous environment. Because organic soils were not widely developed, the earliest terrestrial plants probably first colonized bare rock near large water sources, such as oceans and lakes. Generations of these plants recycling nutrients (e.g., nitrogen, carbon, and oxygen) and energy into the stratum contributed to the development of a rich organic soil suitable for large shrubs and herbs. With the proliferation of these low-lying plants, competition for available space, nutrients, and sunlight intensified. Aerial habitats and those farther afield from the large sources of water represented the only uninhabited environments left to be exploited. This required the physiological and morphological complexity found among the vascular plants.
Cells of the (left) phloem and (right) xylem.
Cells of the (left) phloem and (right) xylem.
Encyclopædia Britannica, Inc.
All plants, including trees, start life as seedlings whose bodies are composed wholly of primary tissues. In this respect, young trees are structurally analogous to the herbaceous plants. It is the conversion of a seedling from an herbaceous plant to a woody plant that marks the initiation of tree-specific structures. In dicotyledonous and coniferous (i.e., woody) trees and shrubs, the defining structure that permits this conversion is a layer of meristematic cells, called the vascular cambium, that organizes between the primary xylem and primary phloem of the vascular cylinders. The cambium forms the wood and the inner bark of the tree and is responsible for thickening the plant, whereas the apical meristems are responsible for forming and elongating the primary plant body. A vascular cambium forms in conifers and dicotyledons and to a lesser extent in some monocotyledons and cycads. Tree ferns do not develop a vascular cambium; hence, no secondary thickening of the trunk takes place in the usual sense.