Plant Cell Types

This lecture on plant cell types derives primarily from a series of slides presented in lecture to support the lecture handout:

Meristematic Cells

Meristematic cells give rise to all three fundamental mature cell types. Their major function is cell division, and so their cell cycle indeed cycles. The walls are thin, the vacuole is largely missing, the plastids are immature, etc. The stages of mitosis and cytokinesis are reviewed. If you are not familiar with these, you should review Chapter 9 in detail.

After mitosis, one of the two resulting cells may mature along one of three developmental directions. I disagree fundamentally with your text author that all derivatives become parenchyma cells and then mature into the other two cell types. But then, I disagree completely with the author on almost all of his summary of parenchyma cells. He has missed the mark on many points (see in Chapter 13 remarks) in my opinion.

Parenchyma Cells

These cells are the biochemistry machines of the plant. They are alive at maturity and are specialized in any number of structural and biochemical ways. Other than support functions, this cell type is the basis for all plant structure and function.

Parenchyma cells have thin primary walls, and highly functional cytoplasm. The cells are alive at maturity and are responsible for a wide range of biochemical function. For example, other than xylem in vascular bundles, the leaf is composed of parenchyma cells. Some, as in the epidermis, are specialized for light penetration, regulating gas exchange, or anti-herbivory physiology. Other cells, as in the mesophyll, are specialized for photosynthesis or phloem loading.

Collenchyma Cells

Collenchyma cells are also alive at maturity and have only a primary wall. These cells mature from meristem derivatives. They pass briefly through a stage resembling parenchyma, however they are determined to differentiate into collenchyma, and this fact is quite obvious from the very earliest stages. Plastids do not develop and secretory apparatus (ER and Golgi) proliferates to assist in the accumulation of additional primary wall. This is laid down where three or more cells come in contact. Areas of wall where only two cells come in contact remain as thin as those of parenchyma cells.

The design and function is to build and maintain the special unevenly thick primary cell wall. The cells are also typically quite elongate. The role of this cell type is to support the plant in areas still growing in length. The primary wall lacks lignin that would make it brittle, so this cell type provides what could be called plastic support. Support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma.

Sclerenchyma Cells

These cells are hard and brittle (as you might expect from the root: scler-. The cells develop an extensive secondary cell wall (laid down on the inside of the primary wall). This wall is invested with lignin, making it extremely hard. Lignin, plus suberin and/or cutin make the wall waterproof as well. Thus, these cells cannot survive for long as they cannot exchange materials well enough for active (or even maintaining) metabolism. They are typically dead at functional maturity...the cytoplasm is missing by the time the cell can begin to carry out its funciton.

Functions for sclerenchyma cells include discouraging herbivory (hard cells that rip open digestive passages in small insect larval stages, hard cells forming a pit wall in a peach fruit), support (the wood in a tree trunk, fibers in large herbs), and conduction (hollow cells lined end-to-end in xylem with cytoplasm and end walls missing).

Sclerenchyma includes the fibers used for making thread and fabric...particularly the fibers from flax that are spun and woven into linen. Review the history of Willimantic, CT.

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