Environmental Engineering Reference
In-Depth Information
well as the tip of new shoots, called apical meristems. It is
these areas that account for the growth in root and shoot
length in most woody plants. In contrast, grasses have meri-
stematic tissues located near the area of leaf attachment to
the stolon or rhizome and are called intercalary, which
explains why the tips of grasses can be removed during
mowing or herbivory without damaging future plant growth.
In either case, this growth pattern is called primary growth, a
common characteristic of both herbaceous and woody
plants, in which the plant increases in length.
Other areas of rapidly growing tissues that require energy
in the form of ATP are the lateral meristems. Rather than
increasing in length, these cells account for an increase in
width, or girth, of a plant; such growth is called secondary
growth. This growth is initiated in the 1-10 cell thick living
tissue called the cambium. In woody plants, these cell layers
are covered by another cell layer called the cork cambium. As
this layer of cells grows, the transport of food and water is cut
off to the epidermal cells causing them to die and be replaced
by a peridermal layer of cells, which becomes the bark of
most plants. In order to reduce water loss through dead cells
that no longer have to the ability to regulate material flow,
before they die these cells secrete suberin, a waxy compound
that acts as a waterproofing agent, into the cell walls.
Compared to prokaryotes, the eukaryotic cell contains
DNA in a separate organelle. The presence of this organelle
in eukaryotic cells was observed through a microscope in
1781 by Felice Fontana. In 1832, this observation was con-
firmed and named the nucleus by Robert Brown
(1773-1858), the Scottish scientist recognized more for his
observation of the motion of tiny particles suspended in
fluids, called Brownian motion. Using his microscope, in
1832 Brown said
In each cell of the epidermis of a great part of this family,
especially of those with membranous leaves, a single circular
areola, generally somewhat more opaque than the membrane of
the cell, is observable, one to each cell. This areola, or nucleus
of the cell as perhaps it might be termed, is not confined to the
epidermis, being found also
Fig. 3.2 The phospholipid bilayer structure of a section of a eukary-
otic cell membrane. The hydrophobic ends of the molecules point
inward and are overlain by hydrophilic heads. This structure helps
explain the round shape of most cells (Modified from Curtis 1983).
water, to cross the membrane easily with no expenditure of
energy by the cell, whereas other compounds are completely
excluded from entry, and some gain entry only after energy
is expended by the cell. Figure 3.2 depicts a linear segment
of the cell membrane, with each end open to the environ-
ment. In reality, this cannot be the case; the ends link
together to form the typical circular shape of most cells
(Smith and Szatham ยด ry 1999). In this manner, a complete
membrane is created and water is retained inside the cell.
The cytoplasm also is filled with other structures called
organelles that help transfer energy and perform other
specialized functions. Mitochondria in the cytoplasm use
the process of respiration, the reverse reaction of photosyn-
thesis, to convert the energy stored during photosynthesis
into work; more specifically, converting reduced organic
compounds in the form CH 2 O into adenosine triphosphate
(ATP) the major form of energy used by the cells. In plants,
mitochondria are concentrated in the actively growing cells
of the root tips, which often have to penetrate impermeable
material of the subsurface. The cytoplasm also contains
other organelles, such as ribosomes, which function to create
proteins from messenger ribonucleic acid (mRNA). Others
organelles include the endoplasmic reticulum, a variable
structure of membranes that serve as sites for chemical
reactions or ribosomal attachment; golgi bodies or
complexes for storage; and lysosomes that contain hydro-
lytic enzymes.
Areas of a plant where cells constantly divide are called
meristems. These include cells at the root tip, or apex, as
in the parenchyma or internal
cells of the tissue. The nucleus of the cell is not confined to the
Orchideae but is equally manifest in many other Monocoty-
ledenous families; and I have even found it, hitherto however
in very few cases, in the epidermis of Dicotyledenous plants.
Robert Brown (1832; in Ford [1985])
...
It was thought that incorporation of the nucleus into
the eukaryotic cell was the result of phagocytosis, the
engulfment of a food particle or prokaryote by the cell, or
the result of symbiosis between different prokaryotes that
provided a selective advantage to both cells. It was later
shown experimentally in the 1930s, using giant marine
algae ( Acetabularia acetabulum ), that the nucleus contained
the information that permitted cell growth and reproduction.
An interesting adaptation unique to plant cells is that all
plant meristematic cells contain the DNA to make itself as
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