30.1: El Cuerpo Vegetal

Habilidades para Desarrollar

• Describir el sistema de órganos de brotes y el sistema de órganos radiculares
• Distinguir entre tejido meristemático y tejido permanente
• Identificar y describir las tres regiones donde se produce el crecimiento de las plantas
• Resumir las funciones del tejido dérmico, el tejido vascular y el tejido del suelo
• Compara tejido vegetal simple con tejido vegetal complejo

Al igual que los animales, las plantas contienen células con orgánulos en los que tienen lugar actividades metabólicas específicas. Sin embargo, a diferencia de los animales, las plantas utilizan energía de la luz solar para formar azúcares durante la fotosíntesis Además, las células vegetales tienen paredes celulares, plastidios y una gran vacuola central: estructuras que no se encuentran en las células animales. Cada una de estas estructuras celulares juega un papel específico en la estructura y función de la planta.

Enlace al aprendizaje

Mira Botany Without Borders, un video producido por la Botanical Society of America sobre la importancia de las plantas.

Sistemas de órganos vegetales

En las plantas, al igual que en los animales, células similares que trabajan juntas forman un tejido. Cuando diferentes tipos de tejidos trabajan juntos para realizar una función única, forman un órgano; los órganos que trabajan juntos forman sistemas de órganos. Las plantas vasculares tienen dos sistemas de órganos distintos: un sistema de brotes y un sistema radicular. El sistema de brotes consta de dos porciones: las partes vegetativas (no reproductivas) de la planta, como las hojas y los tallos, y las partes reproductivas de la planta, que incluyen flores y frutos. El sistema de brotes generalmente crece sobre el suelo, donde absorbe la luz necesaria para la fotosíntesis. El sistema radicular, que soporta las plantas y absorbe agua y minerales, suele ser subterráneo. Figura\(\PageIndex{1}\) shows the organ systems of a typical plant.

Figure \(\PageIndex{1}\): The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil.

Plant Tissues

Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue, and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.

Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.

Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.

Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other (Figure \(\PageIndex{2}\)). In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.

Figure \(\PageIndex{2}\): This light micrograph shows a cross section of a squash (Curcurbita maxima) stem. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue. (credit: modification of work by "(biophotos)"/Flickr; scale-bar data from Matt Russell)

Summary

A vascular plant consists of two organ systems: the shoot system and the root system. The shoot system includes the aboveground vegetative portions (stems and leaves) and reproductive parts (flowers and fruits). The root system supports the plant and is usually underground. A plant is composed of two main types of tissue: meristematic tissue and permanent tissue. Meristematic tissue consists of actively dividing cells found in root and shoot tips. As growth occurs, meristematic tissue differentiates into permanent tissue, which is categorized as either simple or complex. Simple tissues are made up of similar cell types; examples include dermal tissue and ground tissue. Dermal tissue provides the outer covering of the plant. Ground tissue is responsible for photosynthesis; it also supports vascular tissue and may store water and sugars. Complex tissues are made up of different cell types. Vascular tissue, for example, is made up of xylem and phloem cells.

Glossary

apical meristem

meristematic tissue located at the tips of stems and roots; enables a plant to extend in length

dermal tissue

protective plant tissue covering the outermost part of the plant; controls gas exchange

ground tissue

plant tissue involved in photosynthesis; provides support, and stores water and sugars

intercalary meristem

meristematic tissue located at nodes and the bases of leaf blades; found only in monocots

lateral meristem

meristematic tissue that enables a plant to increase in thickness or girth

meristematic tissue

tissue containing cells that constantly divide; contributes to plant growth

meristem

plant region of continuous growth

permanent tissue

plant tissue composed of cells that are no longer actively dividing

root system

belowground portion of the plant that supports the plant and absorbs water and minerals

shoot system

aboveground portion of the plant; consists of non-reproductive plant parts, such as leaves and stems, and reproductive parts, such as flowers and fruits

vascular bundle

strands of stem tissue made up of xylem and phloem

vascular stele

strands of root tissue made up of xylem and phloem

vascular tissue

tissue made up of xylem and phloem that transports food and water throughout the plant