Roots are multicellular organs that absorb water and minerals and keep the plant stationary against external force. Plants like conifers have a long main root called the taproot, which descends vertically into the soil. Lateral branch roots emanate from the central taproot. Some taproot plants, like turnips, store starches and sugars in their taproot for later use during flowering. Grasses lack a taproot and instead have many small roots that grow straight out from the stem; these are called adventitious roots. Root systems of both types have small root hairs that grow out from the roots to absorb nutrients and water.
Just like animals, plants have different organs and specialized tissue types that perform different functions. These different cell types arise from a meristem or region of undifferentiated cells; meristems are found at the tips of shoots and roots and in stems. Plants have three organs, each of which is composed of different types of tissues.
The stem is the second major organ in terrestrial plants; it serves to provide support. Some plants also have modified stems like rhizomes that store food or act as "runners" so the plant can reproduce asexually. One type of vascular tissue, the xylem, carries nutrients through the stem up to the leaves, while the other type, the phloem, carries sugars away from the leaves to other parts of the plant. The points on a stem where leaves are attached are called nodes, and segments in between these points are called internodes. The tip of the stem in a young plant is called the apical bud and is the site where much of the growth takes place.
Leaves are the primary sites for photosynthesis. The vascular tissues in leaves are called veins; leaf veins from different species exhibit many different patterns. Leaves have small pores in their undersides called stomata, which allow the leaf to take up carbon dioxide and exhale oxygen. The tough outer layer of the leaf is called the cuticle. Beneath the cuticle lies a layer called the mesophyll, composed primarily of cells that specialize in photosynthesis. The mesophyll is divided into the palisade mesophyll on top and the spongy mesophyll beneath. A network of air spaces penetrates the leaf to permit circulation of CO2 and oxygen.
- "Biology"; Neil A. Campbell, Jane B. Reece, Lisa A. Urry, Michael L. Cain, Peter V. Minorsky, Steven A. Wasserman, Robert B. Jackson; 2008.
- Estrella Mountain Community College: Plants and Their Structure
- UT Austin: Plant Anatomy
- Photo Credit Ryan McVay/Lifesize/Getty Images
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