Everything Life Sciences Grade 10

Below is an explanation of how guttation occurs:

• At night, when it is dark, transpiration does not occur since the stomata are closed.


• When soil moisture is very high, water will enter the plant roots because the water
  potential of the roots is lower than that of the surrounding soil.


• Thus, water accumulates in the plant, resulting inroot pressure.


• The root pressure forces some water to exit the leaf tip or edge structures calledhy-
  dathodesor water glands, forming drops.


• Root pressure is what drives the flow of water out of the plant leaves, rather than
  transpirational pull.

For guttation to occur there must be a high water content in the soil to create the root

pressure. The transpiration rate must be slow in order for the root pressure to be higher

than the transpirational pull. Guttation must not be confused with transpiration. Table 6.2

highlights the differences between guttation and transpiration.

Guttation Transpiration

occurs early morning and at night occurs during the day when it is hot

takes place through hydathodes takes place through the stomata

Water is lost in liquid form through the

hydathodes

Water is lost as vapour via the stomata

caused by root pressure caused by high water potential

Water droplets are found on the margin

of the leaf

Water vapour transpiration takes place

mostly in the lower surface of the leaf

Table 6.2: Table comparing guttation and transpiration

6.4 Uptake of water and minerals in the roots

In the first section of this chapter, we looked at the structure of the dicotyledonous root and

stem and compared the different cells in the specialised tissues of the plant root and stem.

Now we will look at how these specialised cells help the plant to absorb water from the soil

and transport it to the stem, where it can then be transported to the rest of the plant.

Movement of water through the dicotyledonous root DUMMY

Water is found in the spaces between the soil particles. Water and mineral salts first enter

through the cell wall and cell membrane of the root hair cell by osmosis. Root hair cells are

outgrowths at the tips of plants’ roots (Figure 6.28). They function solely to take up water and

mineral salts. Root hair cells do not perform photosynthesis, and do not contain chloroplasts

as they are underground and not exposed to sunlight. These cells have large vacuoles which

allow storage of water and mineral salts. Their small diameter (5-17 micrometres) and greater

length (1500 micrometres) ensure they have a large surface area over which to absorb water

and mineral salts. Water fills the vacuole of the root hair cell.

Water can now move from the root hair cells and across the parenchyma cells of the cortex

in two major ways. Some water passes through the cells byosmosis. Most water travels

either in, or between the cell walls (of the parenchyma cells) by simplediffusion. The water

180 6.4. Uptake of water and minerals in the roots