Anatomy of Flowering Plants

Anatomy is the study of the internal structure of plants. The plant body is made of groups of similar cells called tissues. Based on whether they can divide, tissues are of two kinds: meristematic and permanent.

Meristematic tissues consist of actively dividing cells and are responsible for growth. By position they are: apical meristem (at the tips of root and shoot — increases length, i.e. primary growth), intercalary meristem (at the base of leaves/internodes, e.g. grasses) and lateral meristem (along the sides — vascular cambium and cork cambium, responsible for increase in girth, i.e. secondary growth).

Permanent tissues are made of cells that have lost the ability to divide and have taken on a fixed shape and function. They are of two types:

• Simple permanent tissues (one cell type): parenchyma (thin-walled, living; for storage and photosynthesis), collenchyma (thickened at corners; gives flexible support to young stems) and sclerenchyma (thick, lignified, dead; gives mechanical strength — fibres and sclereids).
• Complex permanent tissues (more than one cell type, work together for conduction): xylem and phloem, together called vascular tissue.

Xylem conducts water and minerals upward and gives support; its elements are tracheids, vessels, xylem fibres and xylem parenchyma (vessels are absent in gymnosperms). Phloem conducts food (mostly downward); its elements are sieve tube cells, companion cells, phloem fibres and phloem parenchyma (companion cells are absent in gymnosperms).

The cells of a plant are organised into tissue systems: the epidermal (outer protective layer with cuticle, stomata, root hairs), the ground (cortex, pericycle, pith — storage and support) and the vascular (xylem and phloem) tissue systems. Their arrangement differs between organs and between dicots and monocots.

• Dicot root — xylem and phloem in separate radial patches; usually 2–4 xylem groups (tetrarch); has a conjunctive tissue and a pericycle (which gives rise to lateral roots and the cambium).
• Monocot root — similar but with many xylem groups (polyarch) and a large pith; no secondary growth.
• Dicot stem — vascular bundles arranged in a ring, conjoint, open (with cambium between xylem and phloem, allowing secondary growth) and endarch.
• Monocot stem — vascular bundles scattered in the ground tissue, conjoint, closed (no cambium, so no secondary growth), each surrounded by a sheath.
• Leaf (dorsiventral, dicot) — upper and lower epidermis; mesophyll differentiated into palisade (column-like, photosynthetic) and spongy parenchyma; vascular bundles in the veins. Isobilateral (monocot) leaf — mesophyll not differentiated; stomata on both surfaces; some epidermal cells are large bulliform cells.

A simple rule to remember: in dicot stems the vascular bundles are in a ring and open (cambium present); in monocot stems they are scattered and closed (no cambium) — which is why monocots generally show no secondary growth.

Secondary growth is the increase in the girth (thickness) of a stem or root due to the activity of lateral meristems. It occurs mainly in dicots and gymnosperms (most monocots lack it). Two lateral meristems are involved:

• Vascular cambium — forms a complete ring and divides to add new vascular tissue: secondary xylem towards the inside and secondary phloem towards the outside. Far more xylem is added than phloem, so most of a tree trunk is secondary xylem — this is what we call wood.
• Cork cambium (phellogen) — develops in the cortex and produces cork (phellem) on the outside (a protective, waterproof, dead layer that forms the bark) and secondary cortex (phelloderm) on the inside. Small openings in the cork called lenticels allow gas exchange.

In trees of temperate or seasonal climates, the cambium is more active in spring (forming wide, light spring/early wood) and less active in winter/autumn (forming narrow, dark autumn/late wood). One ring of spring wood + one ring of autumn wood makes one annual ring. Counting the annual rings gives an estimate of the age of the tree — a science called dendrochronology.

The older, central wood becomes non-functional, darker and harder (heartwood, which gives mechanical support) while the outer, lighter wood remains functional in conduction (sapwood).