Meristems and plant structure

The plant body Apical Meristems: how the plant grows Cell walls and plasmodesmata Some basic cell types not mentioned yet

Apical meristems Indeterminate growth e.g., many tomato varieties e.g., wheat

Biennial and determinate plants Digitalis purpurea Triticum aestivum Foxglove is a biennial Like many grass species wheat is determinate. It produces a fixed number of leaves and a terminal inflorescence A rosette of leaves is produced close to the ground in the first year of growth

Ricinus: Shoot apical meristem Castor bean

Meristems: how tissues are produced Coleus Apical meristem Transverse section through the apical meristem and newly forming leaves Longitudinal section through the apical meristem

Myriophyllum SEM picture Scanning electron microscope picture of Myriophyllum apical meristem The apical dome is usually convex or flat, as in this example, and its surface is smooth. Myriophyllum SEM picture Developing leaves cover the apex   Most recently budded leaves Water-milfoil

Diagram of shoot apical growth Cell division Elongation zone Differentiation of vascular tissue

Coleus Auxiliary bud meristem The auxiliary meristem may develop into a foliated branch.

Equisetum shoot apex with a single apical cell Equisetum meristem The organization of the meristem is different from Coleus

Root apical meristem

Zea mays root apex Zea mays root apex showing the junction between root apex and the root cap

Lateral root development in Zea mays A meristem develops from parenchyma and the lateral root grows out through the cortex

… structural carbohydrates?

Cellulose

Cell walls and plasmodesmata

Microfibers Electron microscope picture of cellulose-microfibrils in the wall of the green alga Oozystis solitaria Growing plant cells expand through water uptake. In a growing cell enzymes weaken cross-links between the cellulose microfibres of the cell wall allowing it to expand as water flows in by osmosis.

Plasmodesmata Plasmodesmata

Plasmodesmata seen in Section through the cell wall: They are not simple openings but have a complex internal structure.

Some basic cell types 1. Parenchyma 2. Collenchyma 3. Sclerids 4. Bulliform cells

1. Parenchyma Note the nucleus and chloroplasts Young parenchyma tissue cut parallel with the epidermis of Euphorbia pulcherrima (poinsettia). Note the cell contents.  

2. Collenchyma Collenchyma is the typical supporting tissue of the primary plant body. It develops from parenchyma. The cell walls are unevenly thickened. It is common in organs like stems, petioles, laminae or roots. Apium petiole, collenchyma   Apium is celery – and it is the petiole that you eat!

Fig 31.5 B and C

3. Sclerids Thickening of the cell wall Sclerenchyma cells are the principal supporting cells in plant parts that have ceased elongation. Sclerenchyma fibres are the source material for many fabrics, e.g., flax, hemp and jute. Parenchyma Developing sclerid

A sclerid with the cell completely occupied by wall Contrary to collenchyma mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole. Leaf of Podocarpus Note the wall laminations and the Pits around 3 & 5 o'clock. Viewed with polarized light

Fig 31. D

turgor pressure?

Bulliform cells Transverse section of grass leaf Poa praetense. During drought water is lost from the thin walled bulliform cells and the two sides of the leaf blade fold up toward each other so the leaf is less exposed to sunlight and is heated less. Once adequate water is available, turgor increases, and the leaves open again.

Sections you need to have read 4.19, 31.1 through 31.8 Courses that deal with this topic Botany 441 Morphology and anatomy of land plants