Lecture 15

Primary Roots

I. General Functions of the Root System

A.    Absorption of water and minerals from substrate.
B.    Storage of photosynthates.
C.    Anchorage of the plant to the soil.
D.    Control and distribute the the flow of water throughout the plant.

II. Root Systems: Link

A. The Taproot System.  Examples: Amaranthus palmeri, Raphanus sativus. The primary root originates from the radicle of an embryo and gives rise to the taproot. Smaller roots branch off of the main root. Common in Dicots and Gymnosperms.

B. The Fibrous Root System. Example: Setaria (a grass). The primary root gives rise to numerous smaller roots. The primary root then degenerates leaving only the smaller roots.  Common in Monocots.

III. Root Meristems (Allium root L.S.)

A.  Calyptrogen. Meristem that creates and resupplies parenchyma to the root cap. This meristem is adjacent to a dermatogen that produces the epidermis. Thus, the root cap and epidermal cells have independent origins. This type of root organization is found in monocots (image1, image2). In contrast, in dicots the root cap and epidermal cells have a common origin.

B.  Root Apical Meristem. Gives rise to the three primary meristems (protoderm, ground tissue, and procambium). 

C. Protoderm. Forms the dermal tissue system. (Primary tissues include: Epidermis)

D. Ground Meristem. Forms the ground tissue system. (Primary tissues include: parenchyma, collenchyma, & sclerenchyma)

E. Procambium.  Forms the vascular tissue system. (Primary tissues include: xylem and phloem) This tissue system will differentiate into the vascular cylinder.

F. Quiescent center. The root can be examined with respect to where most cell divisions are taking place. (Figure 14.14). The most distal cells of the root, below the apical meristem and above the root cap, divide infrequently. This region is called the quiescent center. It is thought that it facilitates the positional development of primary meristems. Or it is the result of antagonistic forces - surrounded by actively dividing cells, proliferating in different directions.

IV. Meristem Pattern Terminology

A.   Closed Meristems. Vascular cylinder, cortex and root cap cells all develop from their own initials at the root apical meristem and form distinct layers.  Seen in angiosperms such as Stipa and Raphanus (Figure 14.12A & B). Examples: Allium, Linum

B.   Open Meristems. Vascular cylinder, cortex and root cap cells all develop from common initials at the root apical meristem and do not form distinct layers. Seen in gymnosperms such as Picea (Figure 14.12 C). Example: Pinus

V. Developing Root Anatomy

The developing root tip may be divided into three generalized regions or zones in addition to the root cap.

A. The Root Cap.
1. The root cap is a covering that functions in the protection of the root apical meristem and soil lubrication. It is made up of parenchyma that secretes mucigel. The parenchyma cells are short lived and continually replaced from the calyptrogen. Figure 14.4.
2.  Rootcap also orients plant growth using statolith starch grains (Figure 14.11). Statoliths in root cap of cattail (Typha).
3.  Paper on gravitropism, statoliths, and auxin.

B. Developing Root Regions. Root Zones. Shown well on this Amaranthus seedling root.
1.  The Region (or Zone) of Cell Division. Filled with cells actively undergoing mitosis (image). Cells will appear very small. This is the classic mitosis example for most introductory biology slides.
2.  The Region of Elongation. Cells are elongating after undergoing division.
3.  The Region of Maturation. A distinctive feature of this region are the numerous root hairs (Amaranthus) that increase the root surface area.

VI. The Primary Root Structure

A. Water Flow.  The structure of the primary root is highly correlated with controlling the flow of water.
1.  Apoplastic Flow. Water that flows in intracellular spaces and along cell walls. (Fast)
2.  Symplastic Flow. Movement of water from cell to cell through plasmodesmata.  (Slow). Also includes Transcellular Flow. Movement of water from cell to cell and through the vacuole.
3.  Diagram showing paths of water flow (Image)

B. The Primary Root Structures - image1, image2 of Smilax root (a monocot) and image1, image2 of Ranunculus root (a dicot)
1. The Outer Root (Figure14.13) Smilax root X.S.
a.  Epidermis. The outermost tissue of the dermal tissue system. In roots the epidermis tends to have a think cuticle. Some plants will develop an exodermis in order to better control the flow of water.
b.  Exodermis. This layer is not always present but functions in a similar manner as the endodermis. Figure 14.3 from Zea. Photo of Smilax root and closer view.
c.  Cortex. This layer often comprises the majority of the root mass. It is frequently made up of storage parenchyma.
d.  Endodermis. This layer is generally a single cell layer thick and is characterized by suberin impregnated walls, known as the Casparian strip. Diagram of the endodermis Figure 14.5. This layer forces water to flow symplastically allowing for the control of ion movement. 

2. The Vascular Cylinder. Diagram Figure 14.16.
a.   Pericycle. The pericycle is considered to be the outermost layer of the vascular cylinder and functions in secondary growth. It may be one to several layers thick.
b.  Xylem. In roots xylem develops from protoxylem poles inwardly from procambium.
c.  Phloem. Phloem forms bundles intermittent with xylem.

C.    Lateral roots
1.  Lateral roots or branch roots form endogenously from the pericycle and progress through the endodermis, cortex, and epidermal layers.  Figure 14.17 of Helianthus (sunflower) and Figure 14.18 of Bromus (a grass).
2.  Salix lateral root (image1, image2)

VII. Introduction to Steller Patterns

A.  Stele. An arrangement of vascular tissue. Stellar patterns differ in roots and stems.

B.  Stelar Types
1. Protostele has xylem in the center. This is the ancestral condition found in early vascular plants.
a. Haplostele - Xylem in rounded core.
b. Actinostele - Xylem in lobes (Figure 14.9). The protoxylem are towards the outside and the metaxylem to the inside.  This type of centripetal maturation is called Exarch.
c. Plectostele - Xylem scattered but continuous. Found only in Lycophytes (Lycopodium).

2. Siphonostele [will see next time]

VIII.  Specialized Roots

A. Specialized air roots. Have an extra velamen layer, such as in many epiphytic orchids. Roots. Section.

B. Adventitious roots. Roots that originate from a stem for support. Example: Pandanus.

C. Root nodules. Chambers within the root that facilitate plant bacterial interaction. The plant provides photosynthates to nitrogen fixing bacteria witch in turn convert atmospheric nitrogen to ammonium and nitrate.
1. Common in the Fabaceae (Pea family). Sesbania, Lupinus (note red color)
2. Alnus (alder, Betulaceae)

D. Haustorium. A modified root of a parasitic plant that connect to the host plant's vascular tissue. Various haustoria HERE.

E. Fungal Root Interactions. 80% of all plants facilitate some form of fungal root interactions. Mycorrhizal relationships increase the plant’s water/ion absorption area.

F. Mycorrhizae. Come in two basic types:
1. Arbuscular (Endo) mycorrhizae. Form penetrating structures, known as arbuscules, into the root cortex. The fungus absorbs photosynthates from the plant and provides a greater water/ion absorption area for the plant. Example: Corallorhiza (an orchid) that has highly modified roots whose cells contain a mycorrhizal fungus. Closer view of cells.
2. Ectomycorrhizae. Form a filamentous network on the outside of the root (the mantle or fungal sheath) and hyphae that occur in intercellular spaces - known as a Hartig net.
Last updated: 14-Oct-22 / dln