Weed Anatomy, First Edition. Hansjoerg Kraehmer and Peter Baur. © 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

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Chapter 25

Root Histology

H. von Guttenberg (1968) subdivides the root into six tissue layers: rhizodermis, exodermis, cortex,

endodermis, pericambium and vascular cylinder. The term pericambium does not seem to be suitable for monocots as they usually do not develop a root cambium for secondary growth. For this reason the term pericycle will prevail here.

Figure 25.1 shows a transverse section through a young root of Alopecurus myosuroides Huds. with all major tissue areas of a plant root. The cortex of this section appears to be out of focus as some cells have started to disintegrate. This is the case in many well-watered root systems where the availability of oxygen becomes a problem and where aeration systems are

created by disintegrating cortex cells (Vasellati et al. 2001). An advanced stage of a dicot root is shown in Figure 25.2. Sections of dicot roots at a later stage of development with secondary growth can look very similar to stem sections. The root xylem will be dis-cussed in detail in the following Chapters. Primary phloem of grass weeds is located between xylem strands (Figure 25.3) and can be recognised in many cases by three cells forming a pentagon-like element – a trapezoid shaped sieve tube with two companion cells (Figure 25.4). This arrangement is different from the monocot and dicot shoot vascular system. The dicot root phloem is located between xylem ridges as figures in Chapter 26 show.

Figure 25.1Transverse section through a young root of Alopecurus myosuroides Huds.

Cortex

Pericycle

Early metaxylemRhizodermis

Hypodermis(exodermis)

Central vessel

Endodermis

98 Section 4 Stem, Root and Growth

Figure 25.3Transverse section through a root of Sorghum halepense (L.) Pers. showing phloem located between metaxylem elements.

Endodermis

Phloem

Pericycle

Metaxylem

Figure 25.4Transverse section through a root of Phalaris minor Retz. with phloem located between metaxylem elements.

Sieve tube

Endodermis

Pericycle

Metaxylem

Companioncells

Figure 25.2Transverse section through an advanced stage of root development Lamium purpureum L.

Xylem

Endodermis

Phloem Primary cortex