Biology Practice Test: Plant Structure and Function

Xylem is made of hollow, dead cells that form tubes for water and minerals moving upward. Phloem is the other transport tissue, but it moves sugars, not water from the soil.

Phloem carries dissolved sugars from where they are made or stored to where they are used or stored, in either direction. Xylem flow goes only upward from the roots.

Root hairs are thin extensions of epidermal cells, so they add a huge absorbing surface in contact with soil water. They are far too fine to anchor the plant, and roots underground cannot photosynthesize.

Auxin makes stem cells elongate, so the shaded side grows faster and the stem curves toward the light. If the lit side grew more, the stem would bend away from the window.

As water evaporates from the leaves, it pulls on the chain of water molecules in the xylem, which stick together by cohesion. Plants have no pump, and gravity works against upward flow.

Gravitropism is growth in response to gravity: roots grow down and shoots grow up. Phototropism is the response to light, and a buried seed gets no light to respond to.

The anther sits on top of the filament, and together they form the stamen, the male part. The stigma is the sticky tip of the female part that receives pollen, so it is a common mix-up.

The ovary wall swells and ripens into the fruit, while the ovules inside become seeds. Petals usually wither and fall off after pollination.

Parallel leaf veins and flower parts in multiples of three are typical monocot features, as in grasses and lilies. Dicots usually have branching, net-like veins and parts in fours or fives.

Without xylem and phloem, water moves only slowly from cell to cell, which limits their height. They do photosynthesize, and they reproduce with spores, not seeds.