Internal Structure of Flowering Plants and Mammals

If you cut across a stem of balsam or a young maize stalk and look at it with a hand lens, you see a pattern of rings and dots. Each dot is a vascular bundle, a pipeline carrying water up and food down. Roots, stems and leaves are built from the same few tissues, but each organ arranges them to suit its job. In this lesson you will name the main plant tissues and their functions, describe the arrangement of tissues in a dicot root, stem and leaf, contrast dicot and monocot stems, and outline the four basic tissues and main organ systems of a mammal.

Dicot root: the outer piliferous layer carries root hairs that absorb water. Inside is a wide cortex for storage, then the endodermis, a single ring of cells that controls entry of water into the centre. The pericycle lies inside the endodermis and gives rise to lateral roots. At the centre, xylem forms a star shape (often four arms) with phloem between the arms, and there is no pith. Placing the vascular tissue centrally helps the root resist pulling forces. Dicot stem: vascular bundles are arranged in a ring, each with phloem outside, cambium in the middle and xylem inside, around a pith. Monocot stems such as maize have many bundles scattered through the ground tissue, no cambium and no distinct pith, so they do not thicken by secondary growth. Leaf: a waxy cuticle and upper epidermis let light through but reduce water loss. Beneath is the palisade mesophyll, tall cells packed with chloroplasts, then the spongy mesophyll with large air spaces for gas exchange. The lower epidermis has many stomata, each controlled by two guard cells. Veins (vascular bundles) bring water in xylem and remove sugar in phloem; xylem is on the upper side of the vein.

A mammal is built from four basic tissue types. Epithelial tissue covers surfaces and lines cavities, such as the skin and the lining of the gut. Connective tissue joins and supports: bone, cartilage, tendons, ligaments, adipose tissue and blood are all connective tissues. Muscle tissue contracts to produce movement and comes in three kinds: skeletal (voluntary), smooth (involuntary, in the gut wall) and cardiac (in the heart only). Nervous tissue carries impulses. These tissues combine into organs. The stomach, for example, has an epithelial lining, smooth muscle layers, connective tissue and nerves. Organs then combine into systems: digestive, circulatory, respiratory, excretory, nervous, endocrine, skeletal and reproductive. Inside the body, the thoracic cavity (heart and lungs) is separated from the abdominal cavity (gut, liver, kidneys) by the diaphragm.