Chapter 13 Remarks
Remarks on:
Randy Moore et al. 1995. Botany. Wm. C. Brown Publishers. Dubuque, IA.
Cells and Tissues
This chapter, unlike Chapter 9, focuses on mature cell types...what happens after a meristematic cell creates what will be a derivative cell. The derivative cell will differentiate (become different) from its meristematic precursor to go along one of several different pathways of maturation. The three major paths lead to these mature cell types:
- Parenchyma
- Collenchyma, or
- Sclerenchyma
Please do not fall for the misconception that parenchyma becomes collenchyma and then matures as sclerenchyma. That is a wrong idea...a common misconception but nevertheless wrong.
Parenchyma
Parenchyma cells are alive at maturity and are responsible for virtually all of the important an interesting biochemistry of plants. They have thin (primary) walls and are typically (though not always) isodiametric (close to spherical). I take real issue with the author in his opening sentences...
- First, parenchyma cells are not likely the most abundant cells in plants if you look at trees. Clearly the wood and bark of the trunk and branches (mostly sclerenchyma) holds vastly more cells than the leaves you rake up in the autumn. Now if you are going to limit your thinking to just the living cells, then surely parenchyma cells are the most abundant. Maybe that is what he meant.
- Second, "few distinctive structural characteristics" is just plain wrong. When I think of parenchyma cells I think of glandular and other hairs and scales, root hairs, guard cells, subsidiary cells, endodermal cells, palisade mesophyll, and I'm sorry I just don't buy this statement. There are many, many parenchyma cell forms and structures and I'd say that by comparison collenchyma (to a great degree) and sclerenchyma (to a lesser degree) are just plain boring.
From a functional point of view, parenchyma cells are vastly more interesting than the other cell types.
Note that the parenchyma cell typically has a large vacuole. This may account for 90% of the volume and weight of the total protoplast. Thus, when we are contemplating how much tissue to weigh out for a particular project, you don't get as much cytoplasm as you think. First you have to subtract the weight of the vacuole, then subtract the estimate of cell wall weight, then subtract another fraction for the weight of the intercellular fluid. These components together comprise at least 95% of the weight you measure in a typical situation. This adds some challenge to calculating cellular concentrations of components. It definitely makes plant tissue good "diet food" as 90%+ of what you are eating is calorie-free!
Collenchyma
Collenchyma is an elongated cell type. Its walls are exclusively primary, but portions of the wall are specially thickend. The thickenings typically occur at the "corners" of the cells, where three or more cells come in contact with each other. The walls in areas where only two cells touch are typically as thin as any parenchyma cell. This cell is alive at maturity and serves as a kind of plastic support element. It helps young seedlings stay erect, but yet it allows for the stem to continue elongating. Thus, "stretchable" support is its primary role. The biochemistry it carries out deals mainly with its own maintenance...it is usually not photosynthetic, but depends on sugars for respiration to supply its energy needs.
I disagree with the author's comment that collenchyma cells "differentiate from parenchyma cells". This is what leads students into wrong-thinking. An immature collenchyma cell may look something like a parenchyma cell, but so what? This cell is not yet mature and, regardless of its superficial looks (hey, it is elongated), it is destined to become mature collenchyma!
The author talks about shaking plants to induce collenchyma. This process sounds like "seismomorphogenesis" and I'd like to read more about this. Maybe you will want to do an independent study project on this. There are some fun techniques you can use to assess the possibility of this kind of phenomenon. This is the first I have heard of it, so it might be fun to see what we could demonstrate.
Sclerenchyma
Sclerenchyma cells have secondary walls in addition to the primary wall. This makes them very hard as the secondary wall usually contains some lignin, suberin, and cutin. This embellishment cuts off the water and chemical trasnsport processes for the sclerenchyma cell's cytoplasm, and so it generally dies (unless there are good plasmodesmata and/or pits). Yes, this is a class of cells that function best when they have died.
For a human parallel, think of your epidermal cells...yup, 100% dead. We spend fortunes on keeping it wrinkle-free, and just the right color to attract love-interests. Dandruff shampoos purport to "cure" what is an essential process--the shedding of dead cells. If they worked as claimed I'm sure the government would take them off the shelves...you want turnover of these cells! Enough about humans.
One particular class of sclerenchyma is fibers. These cells are quite long (up to a meter or more) and can be twisted together to make thread. This was done right here in Willimantic. Our empty thread mills are the only evidence now of what was a world-renowned industry in our town. The most notable thread made here was linen. The linen thread was made from the fibers of the flax plant. The thread made in Willimantic was woven into linen fabric in yet other factories in our town. Willimantic was the world's leading producer of linen fabric! Now the whole industry is gone. Totalled by polyester in the 1970s and completely eliminated by expensive labor markets in the north-east US, the industry for natural fiber fabrics is making a comeback elsewhere. It is a sad history for us.
Tissues
Plants have three fundamental tissue types:
- Dermal Tissue
- Ground Tissue
- Vascular Tissue
There are some other specialized types, but these three cover most of the basic ones.
Dermal tissues cover the exterior of plant organs. There are epidermal layers on root, stem, and leaf. These exterior layers have many specializations that are nicely covered in you book. Read up.
Ground tissue fills the space between the dermal tissue and the vascular tissues. But it is more than filler. As I mentioned before, this is where virtually all of the interesting biochemistry goes on. This is where virtually all of the photosynthesis occurs, for example.
Vascular tissue is a conducting tissue type. It is divided into two parts:
- Xylem is a tissue that consists of mostly sclerenchyma (dead cells). The cells are connected end-to-end (and/or side-to-side) to serve as "pipes" for water and minerals to be conducted from root to stem to leaf to atmosphere. The flow of water from soil to atmosphere along this xylem pathway is called transpiration.
- Phloem is a specialized parenchyma tissue. The phloem cells have a cytoplasm and are therefore alive. The conducting cells have at least a rudimentary metabolism. They depend on adjacent companion cells for most of the directed metabolism and supply of essential raw materials for life. The materials are passed through plasmodesmata between the cells. The phloem carries sugar, amino acids, and other chemicals from sites of synthesis (usually the leaf) to the rest of the plant. Notice then that flow in the phloem (called translocation) is bidirection (both up and down the plant). Most people think xylem conducts water and minerals up and phloem conducts sugar (only) down (only half-right).
There is a lot of detail to read here and our examinations in laboratory will help you see some of these each week. You might want to re-read this chapter later in the semester to tie it to the anatomy of the leaf, stem, and root.
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