Lecture 2: Plant Growth
Fact that plants are autotrophic, sessile, and have a cell wall have ripple effects on development
Plant development today
Because of cell wall, behavior of individual cell is critical to plant development.
Critical processes related to division
Timing and rate of cell division
Cells must coordinate for perfectly flat leaf. When introduce mutation with uncoordinated cell
division, you get a not flat leaf
Orientation of cell wall formation periclinal vs anticlinal
Tightly controlled orientation of cell division – perpendicular to outer surface of plant. Builds
outward in a continuous layer. Anticlinal.
Periclinal – divide parallel to surface of plant, divide up and down.
Symmetry of cell division
Most plant cells divide in a perfectly symmetrical fashion, but there are some specific times in
development (stomatal development and embryogenesis) there is asymmetric division – in
embryogenesis, first division is asymmetric. Apical cell gives rise to embryo, basal gives rise to
Rate and direction of cell expansion controlled by cooperative interactions between cytoskeleton,
vacuole, and cell wall. Typically plants do all cell division, then they expand. Vacuole swells with
water and pushes outward on cell surface
Two modes of cell growth:
Diffuse growth (wall expansion distributed over cell surface)
Tip growth (wall expansion localized to one end of cell) (happens in very specific cases, like cotton)
There are components in cell wall that restrict expansion in certain directions.
How do plants make shapes other than spheres?
Control what materials are put out into cell wall – control orientation of the cellulose microfibrils. So,
isotropic pressure (turgor pressure) comes from vacuole but because of way that cellulose laid
down controls direction of expansion. Pressure in other direction not constrained – fibrils can slide
away from each other. Composition of cell wall and orientation of cellulose and degree of
crosslinking controls what parts of cell wall can expand.
Plants grow in continual process of organogenesis generated by activity of meristems. Meristem is
population of perpetually dividing, undifferentiated cells
Meristematic tissue occurs in many places: apices (root or shoot), lateral regions (axillary
meristems). Where leaf joins stem is called axle. Lateral regions (vascular cambium and cork
cambium) – control girth. Also intercalary meristem regions. In the stem, cells that are still behaving
like meristems – undifferentiated
Apical meristems can be single cells
Ferns, equisetum, other early land plants.
Seed plants have more complex apical meristems. We can divide meristem into different zones
Central zone = cells divide slowly
Peripheral zone – cells divide rapidly, where new primordial arise, new leaves
Rib meristem – underlie CZ and give rise to vasculature – giving rise to stuff below it in stem
Or, look in terms of layers
L1/L2 – tunica – anticlinally dividing, continuous layer of cells forming epidermis
L1 layer always defined as single layer of epidermis, but L2 may be multiple layers – L1 + L2 =
L3 – corpus – start seeing periclinal divisions on a regular basis
Layers are contiguous throughout the plant
Tissues of plant bod