Roots Fibrous Roots Tap Root Primary Root Growth

Roots Fibrous Roots Tap Root

Primary Root Growth 1 = emerging lateral root, 2 = lateral root primordium, 3 = root hair nearly fully grown, 4 = mature vessel element, 5 = level at which root hairs appear, 6 = endodermal cells differentiate, 7 = level at which elongation ceases, 8 = first vessel element begins to differentiate, 9 = region of maximum cell elongation, 10 = first sieve tube matures, 11 = cell division ceases in most cell layers, 12 = first sieve element begins to differentiate, 13 = pattern of future vascular cylinder becomes recognizable at this level, 14 = zone of maximum rate of cell division, 15 = apical initials, 16 = root hair zone, 17 = growth in length, 18 = elongation zone, 19 = meristem, 20 = rootcap

Root hairs

Dicot root with an actinostele

Development of actinostele

Monocot roots have a polyarch actinostele Wheat Root Lily Root

Corn root with parenchyma in center of the stele

Endodermis • Stage I – Endodermal cells have a Casparian Strip • Stage II – Endodermal cells develop suberin in all wall areas except for plasmodesmata – especially inner tangential wall and radial walls • Stage III – Endodermal cell wall becomes thickened with added layers of cellulose and lignin • Passage cells – Endodermal cells that remain in Stage I condition while neighboring cells are in Stage II or III (often near xylem poles) • Exodermis with similar properties occurs in some species just beneath the epidermis

Endodermal cell with Casparian strip

Endodermis Early stage of root development Later stage of development

Orchid roots are modified by development of a velamen – a multiple epidermis consisting of dead cells – an exodermis separates the velamen from the cortex Velamen Exodermis

Dicot root drawing at end of primary growth 1 = epidermis 2 = root hair, 3 = cortex, 4 = intercellular spaces, 5 = pericycle (one layer colored dark green), 6 = endodermis, 7 = primary phloem, 8 = protoxylem, 9 = metaxylem, 10 = initial vascular cambium, 11 = sieve plate

Branch roots

Development of branch roots from pericycle

Development of branch roots

Branch root

Branch root in corn

Secondary Growth in Roots 1 = primary xylem, 2 = endodermis, 3 = primary phloem, 4 = pericycle, 5 = vascular cambium, 6 = secondary phloem, 7 = secondary xylem

Limited secondary growth in castor bean root

Tilia root

Tilia root periderm

Carrot roots have more secondary phloem than xylem 1 = secondary phloem, 2 = vascular cambium, 3 = parenchymatous woody secondary xylem

Types of Mycorrhizae l Ectotrophic mycorrhizae – Sheathing mycorrhizae l Endotrophic mycorrhizae – VA (vesicular-arbuscule) mycorrhizae

Ectotrophic Mycorrhizae • Sheathing mycorrhizae of forest trees • Roots have club-like appearance • 5000 species of mushrooms form these

Ectotrophic Mycorrhizae

Ectotrophic Mycorrhizae

Hartig Net

Endotrophic Mycorrhizae • Common on herbaceous plants and trees • Mycelium branches in cortex of root forming arbuscles and vesicles

Endotrophic Mycorrhizae

VA Mycorrhizae Arbuscule Spore

Roots and Shoots – What connects the two areas?

Hypocotyl connects the root and shoot starting in the embryo