Angiosperms, or flowering plants, are the largest and most ecologically dominant group of vascular plants on Earth. They are characterized by producing flowers and enclosing their seeds within a fruit, a trait that has contributed significantly to their success. The internal transport system is a highly developed network of specialized tissues running continuously from the roots through the stems and into the leaves. This vascular system allows for the efficient long-distance movement of essential resources throughout the plant body, enabling the evolution of larger, more complex forms.
Defining the Vascular System: Xylem and Phloem
The plant vascular system consists of two distinct complex tissues: xylem and phloem. These tissues are typically found bundled together in strands, forming the vascular bundles that support the plant. The primary role of the xylem is to conduct water and dissolved mineral nutrients upward from the roots. Conversely, the phloem distributes the organic products of photosynthesis, primarily sugars, to areas of growth or storage. Xylem contains tracheids and vessel elements, which are dead and hollow at maturity, while phloem is composed of living sieve tube elements assisted by specialized adjacent companion cells.
Xylem Structure and Water Movement in Angiosperms
The xylem in angiosperms contains both tracheids and vessel elements, with vessel elements being a major specialization. Vessel elements are shorter and wider than tracheids and stack end-to-end to form continuous, pipe-like structures called vessels. The end walls of these stacked cells have perforations, allowing for a much faster and more voluminous flow of water than the restrictive tracheids. Water movement is a passive process explained by the Cohesion-Tension Theory. Transpiration creates a negative pressure that pulls the water column upward, and the wide vessels reduce resistance, supporting high photosynthetic rates.
Phloem Structure and Nutrient Distribution
Sieve Tube Elements and Companion Cells
The phloem tissue is built around conducting sieve tube elements, which are long, cylindrical cells joined end-to-end. At maturity, these elements lack a nucleus and most other organelles, maximizing space for the movement of phloem sap. The end walls between adjacent elements are perforated with pores called sieve plates, facilitating the flow of materials. Adjacent companion cells retain their nucleus and organelles, performing the metabolic functions necessary to keep the sieve tube element alive. They are responsible for actively loading and unloading sugars into the phloem.
The Pressure-Flow Hypothesis
The long-distance transport of sugars is described by the Pressure-Flow Hypothesis. This hypothesis states that sugars are actively loaded at a “source,” such as a leaf, drawing water from the adjacent xylem by osmosis and creating high hydrostatic pressure. This pressure forces the phloem sap to flow toward a “sink,” such as a root or a developing fruit, where sugars are actively unloaded. The removal of sugar at the sink causes water to exit the phloem, maintaining the pressure difference that drives the mass flow of nutrients.
The Evolutionary Advantage of Angiosperm Vascular Tissue
The highly efficient vascular system is a major factor in the ecological dominance of angiosperms. The development of wide, perforated vessel elements allows for significantly faster water transport compared to the narrow tracheids found in non-flowering vascular plants like gymnosperms. This enhanced hydraulic efficiency enables angiosperms to sustain higher rates of transpiration and photosynthesis. The ability to move large volumes of water and nutrients quickly supports a faster growth rate, allowing angiosperms to achieve greater stature and biomass. This size advantage allows them to outcompete other plants for light and resources, while the specialized phloem ensures rapid distribution of photosynthetic products.