Root Structure and Function Penetration of Soil Gravitropism Downward Growth Water and Mineral Intake Conduction (Xylem and Phloem) Storage of Materials Branching Anchorage Zone of Maturation - cell differentiation Protoderm Ground Meristem Provascular Zone of Cell Elongation - cell expansion Notice how the growing zone has no root hairs or lateral roots! Growth among soil particles would result in shear forces. Zone of Cell Division - new cells by mitosis Root Cap - penetration, padding Mucilage Slough Cells Gravitropism Root Tip Senses Gravity Auxin Hormone Produced Auxin Accumulates on Lower Growth Inhibited on Lower… Relative to Upper Root Curves Downward Water and Mineral Uptake Root Hairs Increase Surface Area Root Hairs Secrete Acid (H+) H+ Cation Exchange w/Minerals Mineral Uptake into Roots ©1996 Norton Presentation Maker, W. W. Norton & Company Radish seedlings have roots with long root hairs that increase the surface area for water and mineral uptake Osmosis: passive movement of water from pure to polluted area cell membrane cell wall water flow cytoplasmic solutes more concentrated Water potential low soil solutes more dilute Water potential high Root hairs are responsible for cation exchange cortex cell epidermal cell root hair penetrates soil spaces intercellular gas space Ca2+Ca H 2+ soil particles covered with capillary water and minerals + to vascular cylinder Ca2+ H+ voids with air space water Dicot Mature Root Structure - Anatomy Ranunculus acris - buttercup Epidermis Cortex Vascular Cylinder What does all of this autumn color (leaf senescence) have to do with roots? Root Vascular Cylinder and Cortex Ranunculus acris - buttercup Endodermis Cortex Phloem Metaxylem Endarch: protoxylem is inside the metaxylem Exarch: protoxylem is outside the metaxylem Protoxylem √ Pericycle Specialized Versions Protosteles: haplostele actinostele plectostele siphonostele solenostele dictyostele cortex phloem xylem dicot root cortex phloem xylem pith leaf gap leaf trace monocot root eustele dicot stem atactostele monocot stem Apoplastic ©1996 Norton Presentation Maker, W. W. Norton & Company Symplastic endodermis xylem inside The endodermis is thus responsible for selective mineral uptake. suberinwaxy barrier to apoplastic movement cortex outside minerals cannot go between cells minerals must go through cells cell membrane proteins (active transporters) determine which minerals may be taken up Mineral uptake: Active transport against concentration gradient cell membrane cell wall Calcium transport protein ADP + Pi Ca2+ too expensive? Ca2+ Ca2+ ATP water flow cytoplasmic solutes more concentrated Water potential low soil solutes more dilute Water potential high Osmosis: passive movement of water from pure to polluted area Root Anatomy: Dicot Root Cross Section Epidermis - root hairs, mineral and water intake Cortex - storage, defense Endodermis - selective mineral uptake Casparian strips in radial walls Pericycle - lateral root formation (periderm) (Vascular Cambium - makes 2° tissues) Phloem - CH2O delivery from leaves Xylem - conduct water and mineral upwards One Vascular Cylinder (Phloem + (Cambium) + Xylem) Radial Xylem-Phloem Arrangement Exarch Xylem Maturation Tetrarch (this example) Monocot Root Cross Section Pith Xylem/Phloem Arr? Xylem Maturation? Smilax-catbrier ______arch? How is this section different? Smilax - catbrier A closer look… What do these features tell you? Starch Cutin/Suberin Mitochondria Sieve Tube Element Companion Cell Vessel with Lignin Xylem Parenchyma Lignified Pith Parenchyma What is the Pericycle doing? Root Cap Growing out through cortex Zone of Cell Division In fibrous root systems, there is much lateral root formation. Here you can see two root apices initiating from the pericycle. Notice their connection to the ridges of xylem But shrubs also generally have some compromise for uprooting forces…feeder roots extending laterally. In shrubs like this tea plant (Camellia sinensis), the root system will be more tap root than fibrous root. Notice the diameter of this tap root compared to this man’s waist! Tropical soils are nutrient poor. Roots must traverse the surface for minerals, so roots grow on the surface (no tap root). So, to keep this tall baobab tree standing upright, the roots grow in diameter but only in the vertical dimensions to form ridge roots…called buttress roots. My wife here is as large as I am so you can see these roots are a meter tall! These roots inspired gothic cathedral architects to design buttress walls. http://www.oxc.com.hk/raoul_nathalie/gallery/images/04%20Buttress.jpg http://www.dublincity.ie/dublin/citywalls/buttress.jpg Prop roots such as these inspired flying buttresses. Pandanus utilis - screw pine http://williamcalvin.com/BHM/img/FlyingButtressND.jpg http://www.contrib.andrew.cmu.edu/~ajm/Pages/Graphics/flyingbuttress.JPG Avicennia germinans (black mangrove) pneumatophores