Lecture 5

Parenchyma and Collenchyma


I. Genera information

A. The two major cell types with primary walls: parenchyma and collenchyma. Parenchyma is the main cell type found in ground tissue (e.g. cortex and pith) as well as the mesophyll of leaves and sometimes mixed with other tissues such as xylem and phloem. Ultrastructure of mesophyll cell of Prunus HERE.

B. Parenchyma cells have a variety of functions including photosynthesis, storage, and meristematic activities.  The latter shows that parenchyma cells, being relatively unspecialized, are plastic and can differentiate into other cell types.  These meristematic activities include wound healing, formation of adventitious roots, new shoot systems, and joining of graft tissues. Movie of a shoot apex showing apical meristem.

C.  Parenchyma cells are totipotent, i.e. capable of giving rise to an entire plant, like a fertilized egg (zygote).

II. Parenchyma

Word derived from Greek “para”, beside, and “en-chein” to pour, aluding to the concept that parenchyma is tissue poured beside the other [vascular] tissue.

A.  Living cells that usually have thin evenly thickened 1˚ wall. Is some parenchyma cells the wall is thick and used for storage of carbohydrates (e.g. Diospyros, Coffea, Asparagus and Phoenix). If a tissue is composed totally of parenchyma cells then it is called a parenchyma tissue.

B.  Shapes. Parenchyma cells are generally polyhedral = many sides.  Coleus parenchyma HERE.
1. Some have 14 sides, like a soap bubble, which is called an orthic tetrakaidecahedron. Get your cut-out model HERE, after Matzke (1935). 
2. Parenchyma cells can take on many other shapes as is shown in Fig. 5.1 and Fig 5.2.
3. The orthic tetrakaidecahedron geometrically a truncated octahedron (Wikipedia page) and one of many kinds of tetradecahedrons (Wiki).

C. Types of parenchyma

Page with lots of images of parenchyma from Curtis et al. (2002) HERE
1. Synthetic parenchyma - cells that synthesize something
a. Photosynthetic (chlorenchyma) found in the mesophyll of some stems, young stems, succulent stems, primitive plants, herbaceous plants. Often have a conspicuous vacuole and the cells have air spaces between them (lacunate).
  • Cross section of Nerium leaf showing pallisade and spongy mesophyll HERE
  • Potamogeton mesophyll HERE
  • Video of Elodea leaf chlorenchyma showing cytoplasmic streaming HERE
b. Meristematic - simply diving cells, may differentiate into both collenchyma and sclerenchyma or may stay as parenchyma
c. Secretory - may secrete substances.  Have dense protoplasts. Have lots of ribosomes, ER, or Golgi depending upon the product they are secreting.

2. Structural parenchyma
a. Non-photosynthetic ground tissue, cortex and pith
b. Aerenchyma - special kind of structural ground tissue with lots of air spaces, important in aquatic plants for buoyancy and gas exchange, also spongy mesophyll. These spaces are of schizogenous origin, i.e. the cells separate along their middle lamella.  Some take on a lobed or “armed” form.
  • Leaf of Nymphaea HERE, petiole X.S. HERE
  • Aerenchyma cells of Hymenanche HERE
  • Rhizome of Acorus HERE
  • Stem of Potamogeton HERE
  • Many images from CUPAC HERE
c. Ethylene accumulates in waterlogged tissue, and this gas induces programed cell death (= apoptosis in animals) and the formation of aerenchyma. It may also form constitutively with no environmental stimulus e.g. rice roots.

3. Boundary parenchyma
a. epidermis - outer layer of an organ. Here Psilotum epidermis with thick cuticle and chlorenchyma below.
b. endodermis - inner most layer of root cortex and some pteridophyte stems (e.g. Psilotum, Lycopodium) that has a Casparian strip around each cell (suberin): controls what enters vascular cylinder (stele)
c. exodermis - like an endodermis but directly under the epidermis of some roots (sometimes is sclerenchyma in Smilax)

4. Transport parenchyma
a. phloem - living conducting cells, unlike xylem conducting cells that are dead at maturity. Includes sieve tube elements and companion cells.
b. transfer cells with wall ingrowths called a labyrinth. In a variety of tissues where active transport or facilitated transport is required, i.e. transport of solutes over short distances.  Increases surface area including cell membrane area. Examples:
• parenchyma around phloem conducting cell. Cucurbita stem HERE.
• in bryophytes, the placenta is the zone of contact between the gametophyte and the embryo; transport usually from former to latter. Takakia sporophyte and gametophyte HERE. Sporophyte foot in gametophyte of Anthoceros (a hornwort) HERE.
• conducting parenchyma in bryophytes (Polytrichum stem HERE) and other plants
• seed coat cells in contact with cotyledons in bean (Vicia faba)
• haustorial cells in parasitic plants such as Cuscuta. Figure from Dawson et al. (1994).

5. Storage parenchyma - variety of tissues, e.g. tubers, fruits. In starch storing parenchyma cells, the plastids become amyloplasts. Root cortex of Ophioglossum HERE.  In petals of flowers, the plasids become chromoplasts. 

6. Wood (and bark) parenchyma (Figure): secondary tissues with primary parenchyma. Acer wood HERE.
a. rays parenchyma (Figure):  brick wall arrangement in radial section, tangential section - each circle is a cell oriented laterally
b. axial arranged longitudinally in the trunk, opposite to the rays - oriented longitudinally

II. Collenchyma

Word derived from Greek for “glue”, referring to the glistening walls of these cells.

A.  Like parenchyma, collenchyma has a protoplast capable of resuming meristematic activity.  Distinguished from parenchyma by the presence of a thick, non-lignified 1˚ wall and because they are generally longer than parenchyma cells. Walls have lots of pectin, which holds water.  Treating collenchyma cells with alcohol dehydrates them, causing wall contraction. Image of Apium (celery) parenchyma next to collenchyma.

B.  Collenchyma walls are non-lignified, pliable 1˚ walls that are stretchable but strengthened, present in young organs with rapid expansion.  Compare with sclerenchyma that has more rigid lignified 2˚ walls. Many sclerenchyma cells lose their protoplasts at maturity and are thus not capable of becoming meristematic.

C.  Distribution in the plant (Figure)
1.  Collenchymatous parenchyma – cells with thick walls associated with the vascular bundle (on both outer and inner sides).  Esau does not recognize this a true collenchyma
2.  Independent peripheral collenchyma.  One or a few layers below epidermis.  May form a continuous layer around circumference of axis. May form ridges on herbaceous stem, petioles such as celery HERE, and ribs of larger leaf veins. Rarely occurs in roots.

D.  Function
1. Support of growing leaves and stems. Cells walls thicken during development, but remain plastic and capable of extension
2.  Support in older plant organs that do not have much sclerenchyma. Collenchyma can change to sclerenchyma in older plants by deposition of 2˚ lignified wall.
3. Walls thicker and form earlier in plant parts under stress
4.  Cells are long and overlapping and may reach 2 mm in length, with high tensile strength, thus comparing to fibers (2˚ tissue)

E. Type of collenchyma

Page with images of collenchyma from Curtis et al. (2002) HERE
The three types of collenchyma (from Evert) HERE
1. Angular - walls thickened where cells meet in the corners. Seen in stems of Atropa and Solanum tuberosum and petioles of Ampelopsis, Apium, Begonia, Beta, Boehmeria, Coleus, Ficus, Helianthus, Morus, Pelargonium, Polygonum, Rumex, Ricinus, and Vitis.

2. Lamellar or plate – thickened on tangential walls. Cells appear to line up in rows just below epidermis.  Seen in stems of Eupatorium, Rheum, Rhoeo, Sambucus, and Sanguisorba.

3. Lacunar, i.e. with a hole - thickening around the spaces between the cells. Hole may be closed later by pectic substances.  Seen in stem cortex of Lactuca, Prunella, Salvia, as well as various Asteraceae and Malvaceae.
Last updated: 10-Oct-22 / dln