2) A pressure develop in the solution when it is separated from pure water by a sem
membrane.
3) The division of Cytoplasm.
Answers
Answer:
- Osmotic pressure
- Cytokinesis
Answer:
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Explanation:
Evolutionary changes were necessary for plants to inhabit land. Aquatic plants obtain all their
resources from the surrounding water, whereas terrestrial plants are nourished from the soil and the
atmosphere. Roots growing into soil absorb water and nutrients, while leaves, supported by a stem
superstructure in the aerial environment, intercept sunlight and CO2 for photosynthesis. This division
of labour results in assimilatory organs of land plants being nutritionally inter-dependent; roots
depend on a supply of photoassimilates from leaves, while shoots (leaves, stems, flowers and fruits)
depend on roots to supply water and mineral nutrients. Long-distance transport is therefore a special
property of land plants. In extreme cases, sap must move up to 100 m vertically and overcome
gravity to rise to tree tops.
Karri (Eucalyptus diversicolor) forest in Pemberton, Western Australia. Karri may
reach the height of 80 m, and is the second highest hardwood tree in the world
(Photograph courtesy A. Munns)
This chapter explains the mechanisms by which water can rise to the top of a tall tree, and the
cellular processes essential for plant cells to maintain turgor.
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3.1 - Plant water relations
Figure 3.1 Surface view of cleared whole mount of a wheat leaf showing large and small parallel veins (mauve) with
transverse veins connecting them. Lines of stomates (shown by the orange colour of the guard cells) lie along the flanks
of these veins. Water evaporates from the wet walls of mesophyll cells below the stomates, drawing water from the veins.
Distance between veins is 0.15 mm; scale bar is 100 µm. (Photograph courtesy M. McCully)
Water is often the most limiting resource determining the growth and survival of plants. This can be
seen in both the yield of crop species and the productivity of natural ecosystems with respect to
water availability.
The natural distribution of plants over the earth’s land surface is determined chiefly by water: by
rainfall (R) and by evaporative demand (potential evapotranspiration, PE) which depends on
temperature and humidity. This leads to such diverse vegetation groups as the lush vegetation of
tropical rainforests, the shrubby vegetation of Mediterranean climates, or stands of tall trees in
temperate forests. Climates can be classified according to the Thornthwaite Index: (R−PE)/PE.
Agriculture also depends on rainfall. Crop yield is water-limited in most regions in the world, and
agriculture must be supplemented with irrigation if the rainfall is too low. Horticultural crops are
usually irrigated.
Plants require large amounts of water just to satisfy the requirements of transpiration: a large tree
may transpire hundreds of litres of water in a day. Water evaporates from leaves through stomates,
which are pores whose aperture is controlled by two guard cells. Plants must keep their stomates
open in order to take up CO2 as the substrate for photosynthesis (Chapter 2). In the process, water is
lost from the moist internal surfaces of the leaf through the stomatal pores (Figure 3.1). Water loss
also has a benefit in maintaining the leaf temperature through evaporative cooling.
The ratio of water lost to CO2 taken up is around 300:1 in most land plants, meaning that plants must
transpire large quantities of water on a daily basis in order to take up sufficient CO2 for normal
development.
In this section we will examine plant water relations and the variables that plant physiologists use to
describe the status and movement of water in plants, soil and the atmosphere.
3
One of the challenging aspects of understanding plant water relations is the range of pressures from
positive to negative that occur within different tissues and cells. Positive pressures (turgor) occur in
all living cells and depend on the semipermeable nature of the plasma membrane and the elastic
nature of the cell walls. Negative pressures (tensions) occur in dead cells and depend on the cohesive
strength of water coupled with the strength of heavily lignified cell walls to resist deformation. These
play an important role in water transport through the xylem.
3.1.1 - The power of turgor pressure
Well-watered plants are turgid, and their leaves and stems are upright and firm, even without woody
tissue to support them. If water is lost from leaves through the stomates at a faster rate than it is
resupplied by roots, then plants wilt.
Figure 3.3 A bowl of limp lettuce plants (left) and a day after watering (right) showing the function of turgor in maintain
leaf shape.
Well-watered plants are turgid because their cells are distended by large internal hydrostatic pressures.
(Figure 3.4). The difference between a crisp lettuce leaf and a limp one is that the cells of the limp leaf
are not distended; the pressure inside them is that of the atmosphere; they have zero turgor pressure,
the term used for the hydrostatic pressure of the cells’ contents.