Video Transcript
A cross section of an herbaceous
plant stem is taken. Which of the following substances
is not present in the inner layers? (A) Lignin, (B) starch granules,
(C) cellulose, or (D) suberin.
Herbaceous plants are vascular
plants that have no persistent woody stems above ground. An example of an herbaceous plant
is a dandelion. Its stem remains soft and
succulent, and its above-ground parts die after flowering.
Herbaceous plants are vascular
plants, meaning they have a vascular transport system consisting of xylem and phloem
cells. You might remember that xylem
transports water and minerals from the roots to the rest of the plant. And phloem transports the sugars
produced during photosynthesis from the leaves to the rest of the plant. In herbaceous plants, the xylem and
phloem can be found in the inner layers of the stem. Now, let’s discuss the answer
choices to figure out which one is not present in the inner layers of an herbaceous
plant stem.
Lignin is a polymer that is
deposited to some of the plant’s cell walls to mainly provide mechanical
support. When lignin is deposited into cell
walls, it makes them waterproof and more rigid. Now, take a moment to consider
where it might make sense for the plant to deposit lignin. If you guessed the cell walls of
the xylem, you are correct. Here, lignin prevents water from
leaking out of the xylem vessels and helps them to form an upright, continuous
column to increase the efficiency of water transport. Since xylem vessels are found in
the inner layers of herbaceous stems, lignin will be found there too, and we can
rule out this answer choice.
Starch granules are the main form
plants use to store the organic carbon produced during photosynthesis. Starch is transported through the
plant by the phloem vessels to be stored in underground structures like roots and
tubers, as well as in seeds and stems. Since starch granules would be
found in the inner layers of herbaceous plant stems, either in storage or while
traveling through the phloem to be stored elsewhere, we can eliminate this
answer.
All plant cell walls are primarily
made out of the carbohydrate cellulose. This strong, insoluble polymer
maintains cell turgidity and prevents plant cells from bursting when they take up
excess water. Since an herbaceous stem cross
section contains many plant cells, all of which have cellulose in their cell walls,
we can eliminate this answer as well.
Cork is an impermeable substance
that makes up the outermost layer of bark in woody stems and roots. It lends physical support and
protects against water loss and the entry of disease-causing microorganisms. The main component of cork is a
waxy substance called suberin. It makes woody stems and roots
waterproof and gives cork its characteristic buoyancy. Because herbaceous plants do not
have woody stems or roots and cork is found in the outer, not inner, layers, we
would not find suberin in the inner layers of an herbaceous stem. Therefore, the correct answer is
(D), suberin.
