Lecture 6

Sclerenchyma

I.  General Features

A. In addition to collenchyma, sclerenchyma is one of the principal supporting tissues in a plant.  It is composed of cells that are dead at maturity with thick 2˚ walls that are often lignified. The thick cell wall is often laminated, showing it was laid down in layers.  Remember that primary cell walls have cellulose microfibrils arranged in a random manner. In contrast, secondary cell walls have microfibrils in a highly organized manner, deposited in layers with different pitches.

B. Sometimes parenchyma cells can become sclerified. Thus the distinction between these and sclerenchyma (sclereids and fibers) is not great – the main difference being the presence or absence of a living protoplast.

C. Sclereids and fibers are non-conducting tissues that add hardness or rigidity to tissues

D.  During their development, sclereids and fibers invade intercellular spaces and even push their way between adjacent cell walls. Figure Esau. Two types of growth can be recognized:

1. coordinated growth – when one cell divides, others near it divide to keep pace with tissue expansion; contiguous cell walls and plasmodesmatal connections remain intact.

2. intrusive growth – cell growth that is not coordinated with adjacent ones; results in growing cell occupying cell spaces; no new plasmodesmatal connections formed. Pits in sclereids indicate unexpanded part has remained in contact with adjacent cells.  Branching at tips of fibers and sclereids indicates the growing cell encountered an obstruction.

II.  Sclereids

A.  Distribution.  Sclereids are distributed in a variety of tissues but particularly in stems, leaves and fruits / seeds.

B. Sclereids are sclerenchyma cells that tend to be isodiametric or irregular in shape, or if elongated they branch or run in direction other than with the majority of the cells. They may occur as idioblasts (isolated in other tissues) or in layers.  In this slide of a Nymphaea (water lily) leaf, there are idoblastic astrosclereids in the aerenchyma as well as columnar sclereids in the palisade mesophyll.

C.  Types of sclereids. Figure Esau.
1. Brachysclereids, also called stone cells
• In pear (Pyrus) fruits, the grittiness of the flesh occurs because of groups of stone cells. They are isodiametric cells with ramiform pits.  Stained brachysclereids. Brachysclereids seen with a fluorescent microscope.
• The commonly planted Bradford pear (ornamental tree) has small fruits packed with brachysclereids.
• Stem LS of Ambrosia (ragweed) showing brachysclereids with pit pairs.
• Stem of Hoya (wax plant). Sclereid with polarizing light.

2. Macrosclereids - rod cells, elongate and columnar, occur in layers, e.g. seed coats and outer layers of fruits.
• These macrosclereids are seen in the outer seed coat of Phaseolus. Another view of the purple-stained macrosclereids. What these cells look like separated in a maceration.
• Illustration from Evert.

3. Osteosclereids - bone cells (because of their shape), columnar with enlarged (or branched) ends; includes columnar sclereids that may occur as idioblasts in leaf mesophyll; often occur on inner layer below the macrosclereids. Common in seed coats and outer layer of fruits; very common in legumes.
• "Hourglass cells" seen below the macrosclereids of Phaseolus. Here is the osteosclereids are fused to the seed coat in Morus alba (white mulberry).
• The same in Cucurbita seed coat.
• Massive osteosclereid in Castalia (misspelled name?)

4. Asterosclereids - star shaped, very common in leaves in the mesophyll, restricted to spongy layer. Idioblasts, often have crystals in the walls as calcium oxalate.
• Astrosclereid from petiole of Camellia.
• Astrosclereid from Monstera
• Astrosclereid from Pseudotsuga menziesii (Douglas fir).
• Astrosclereid from leaf of Nymphaea.
• Astrosclereid from leaf of Nuphar.
• Illustration from Evert.

5. Trichosclereid - hair like, elongated and branched, usually in idioblast in leaves.
• Trichosclereid from clearing of a Musa (banana) leaf.
• Illustration from Evert.

6. Filiform sclereid - elongated, fiber-like, rarely branched, common in leaves
• Scale leaves in Allium sativum (garlic).
• Filiform sclereid in leaf of Olea europaea (olive)
• Acicular sclereids from leaf of Pereskea pitatache (a leafy cactus)
• Illustration from Evert.

III. Fibers

A. Fibers are highly elongated, typically occurring in groups and running longitudinally in the root and stem. In the leaf they are oriented the same direction as the veins.  Shown here is a Tilia stem cross section showing the fibers have thick walls and a small lumen (open space).

B. Types of fibers
1. Xylary fibers = wood fibers (here of Quercus).  We will cover these types of fibers in a later lecture (Chapter 8 in Esau)

2. Extraxylary fibers

a. main function is to provide strength and support

b. occur outside the xylem, often in the phloem or around the veins as a cap or a sheath, or in the cortex or ground tissue
• In the stem of Aristolochia.
• In the rhizome of this fern Gleichenia, the fibers occur in the cortex.
• In Ammophila (Poaceae) the fibers occur in the bundle sheath layer that surrounds the vascular bundle.

c.  primary phloem fibers occur on the outer periphery of the vascular bundles near the phloem
• In Linum (flax). Figure from Esau.
• In Helianthus (sunflower)

d.  Both 1˚ and 2˚ phloem fibers occur in many plants. Figure from Esau
• Sambucus (here showing pit pair)
• Malva (Malvaceae)
• Magnolia,Tilia, Liriodendron, Vitis, Robinia
• gymnosperms such as Sequoia and Thuja

e. During elongation, fibers become multinucleate. Some fibers and sclereids divide mitotically after the secondary wall is layed down – then form partitions recognized by the presence of septations. Figure Esau.

C. Economic Fibers
1. Soft fibers

a. Cannabis sativa (hemp)
b. Linum usitatissimum (linen or flax)
c. Gossypium hirsutum (cotton) - fiber from epidermal trichomes of seeds [not like fibers we are dealing with today)

2. Hard Fibers - in many monocots
a. Yucca glauca (Agavaceae). Bast fibers on each side of vascular bundle.
b. Agave sisalana (sisal)
c. Sansevieria (bowstring hemp or mother-in-law tongue)
d. Musa textilis (banana)
e. Ananas comosus (pineapple)
f.  Phormium tenax (New Zealand flax or NZ hemp). Cross section of leaf showing fibers used by Maori to make bags, mats, clothing, sandals, fishing nets, rope, etc.
Last updated: 10-Oct-22 / dln