Lecture 16

Roots – Secondary Growth

Like stems, secondary growth in roots originates from a vascular cambium and phellogen. This is true for gymnosperms and dicotyledonous angiosperms, but not monocots.

I.  Common Types of Secondary Growth

A.  Overall process (Figure 15.1A-H)

1.  Vascular cambium forms from undifferentiated procambial cells in stele – between the primary xylem and primary phloem 
2.  Pericycle cells also participate – later form vascular cambium
3.  Secondary xylem produced inwards, secondary phloem outwards; vascular cambium assumes a circular shape in XS around the stele.
4.  Cambium from the pericycle forms rays, often opposite the xylem ridges. Rays may be absent (Figure 15.3A).
5.  Periderm forms later from the pericycle. Involves both periclinal and anticlinal cell divisions. This forces the cortex, endodermis, and epidermis outward, eventually sloughed off.
6. Phellogen forms in the outer part of the pericycle.  It may later form in deeper layers, thus forming a rhytidome.

B.  Herbaceous Dicot Species.  Listed below are a variety of herbaceous dicot species. Use these to compare and contrast their root noting their similarities and differences.

1.  Alfalfa (Medicago sativa). Figure 15.2A-C.
2.  Clover (Trifolium hybridum). Image1, Image2
2.  Tomato (Solanum lycopersicum). Figure 15.3A, Image1, Image2
6.  Potato (Solanum tuberosum). Figure 15.3D.
x.  Tobacco (Nicotiana tabacum). Image1, Image2
3.  Cabbage (Brassica oleracea). Figure 15.3B. 
4.  Pumpkin (Cucurbita pepo). Figure 15.3C.  
5.  Cucumber (Cucumis sativus). Image1, Image2
7.  Flax (Linum usitatissimum).  Image
9.  Cotton (Gossypium hirsutum).  Image1, Image2, Image3. Note the presence of pith rays, also known as medullary rays.

C.  Woody Dicot Species

1.  The same differences seen in secondary xylem between gymnosperms and woody dicots are seen in the roots, e.g. vessels vs. tracheids.  In general, roots tend to have a higher proportion of parenchyma cells than stems (e.g. higher and wider rays) and vessels with larger diameters.
2.  Examples:
 3.  What are the histological differences between stems and roots?  In some trees, the distinction between these two organs is not sharp.  And environmental conditions can strongly influence whether an organ takes on stem vs. root characteristics.  Halle (Figure 59) shows what happens when a vigorous tree in winter condition is planted upside down.  Plants are incredibly plastic with regard to development!

D.  Root grafting (Image).  Very common among forest trees.  Can envision all the trees connected via an underground network.  Explains why stumps survive after top is removed.  Figure 15.5 shows graft union in Ficus (fig) roots.  The proliferating parenchyma between the two roots forms a new cambium, connects pre-existing ones.

II.  Variations in Root Secondary Growth

A.  Herbaceous Dicots

1. Plants with limited secondary growth.  In Actaea (Doll’s eyes, Ranunculaceae), the cortex and endodermis are retained and a superficial periderm is formed.  Image.
2. In Convolvulus (morning glory, Convolvulaceae), the endodermis is crushed. 
3. In Citrus, the periderm is first formed under the epidermis. Only later does a new periderm form from the pericycle. 
4. Some families such as Rosaceae, Myrtaceae, Onagraceae, Hypericaceae form a polyderm from the pericycle.


B.  Storage Roots

1. Carrot (Daucus carota).  Parenchyma abundant in the xylem and phloem. Image.
2.  Parsnip (Pastinaca sativa). Image1, Image2, Image3,
2.  Beet (Beta vulgaris).  Has anomalous growth in the storage root.  Forms supernumerary cambia around the vascular cylinder.  Figure 15.6A & B.  The vascular cambium produces xylem and phloem and storage parenchyma. Young root (Image), older root showing first vascular cambium (Image), older root with supernumerary cambia (Image), closer view of successive cambia (Image), closer view of one growth layer and vascular bundle with secondary xylem and phloem (Image).
3.  Sweet potato (Ipomoea batatas).  Figure 15.7A & B. Forms anomalous cambia around xylem. Have lots of storage parenchyma permeated by vascular tissue.
4.  Radish (Raphanus sativus). Figure 15.7C & D.


III.  Physiological Aspects

A.  Root vascular cambium activity tied to signals received from the shoot.  Day length (photoperiod) is important.  Concentrations of sugars, auxins and cytokinnins associated with increased activity, but this differs between various species.

B.  In trees becoming active in spring, the buds are the first to be activated and this propagates downward to the stem and eventually the roots. In Pinus, the termination of activity in the fall is basipetal, i.e. starts at root tip and moves inward and upward to the major roots (really, same direction as shoots, just inverted in the root system).

IV.  Adventitious Roots

A.  Definition: Roots arising from stems OR from old roots with secondary growth.

B.  Distinction between adventitious roots and lateral roots not always sharp. Lateral roots usually arise in acropetal succession on roots with primary growth.  Adventitious roots arise from many locations – nodes (Arachis, Hydrocotyle), internodes (Hydrangea, Struthanthus, Pothos), plantlets (Kalanchoe), phyllocladous stems (Selenicereus), and excised leaves (Piper, Saintpaulia). The latter is used in horticulture to propage plants.  In monocots, they proliferate from the taproot and stem.

C.  May form as a result of damage or pathology.  In mycoplasma infected tomato, arise from phloem parenchyma (Figure 15.8). Also can form in excised parts, tissue culture, preformed primordia.

D.  Origin and growth the same as lateral roots.  Form from interfascicular parenchyma or vascular rays near the cambium.

E.  Pruned roots can resprout new ones called replacement roots. Callus forms over damaged area. New roots arise under the wound tissue, initiated in the pericycle opposite the protoxylem ridges.


Last updated: 14-Oct-22 / dln