Plant Biology

Plant biology is the study of how plants live, grow, and reproduce. Plants make their own food by a process called photosynthesis. It takes place mainly in the leaves, inside organelles called chloroplasts. The green pigment chlorophyll absorbs sunlight. Plants take in carbon dioxide (CO2) from the air and water from the soil, and use light energy to make glucose and release oxygen.

The main parts of a plant each have a job. The roots absorb water and minerals from the soil. The stem supports the plant and carries materials up and down. The leaves make food. The flower is the reproductive part of the plant. The anther, which is part of the stamen, produces pollen. Pollen contains the male reproductive cells.

Plants have two types of transport tissues. Xylem carries water and dissolved minerals from the roots up to the leaves. Phloem carries sugars made in the leaves to the rest of the plant. Together they form the plant’s vascular system.

Leaves have tiny pores called stomata (singular: stoma) that allow gas exchange. Each stoma is surrounded and controlled by a pair of special cells called guard cells. Guard cells are unusual among leaf epidermal cells because they contain chloroplasts. Most other epidermal cells do not. In most plants, guard cells are bean-shaped (kidney-shaped). In grasses (monocots), guard cells are dumbbell-shaped. The loss of water vapor from the leaves, mostly through stomata, is called transpiration.

Transpiration helps pull water upward through the xylem. This upward pull is called the transpiration pull. According to the cohesion-tension theory, water rises in tall trees mainly because of two things. Water molecules stick together (cohesion), and the evaporation of water from the leaves creates a tension (pull) that draws the water column up through the xylem.

Flowering plants are divided into monocots and dicots. A monocot seed has one cotyledon (seed leaf), and a dicot seed has two. Monocot leaves usually have parallel venation and monocots have a fibrous root system. Dicot leaves usually have reticulate (net-like) venation and dicots usually have a taproot system.

Plant hormones control growth. Auxin is mainly responsible for cell elongation and for the bending of shoots toward light (phototropism). Gibberellin promotes stem elongation and seed germination, and it is linked to the “bolting” (rapid stem growth) of plants. Germination is the process by which a seed develops into a new plant.

Plants can also reproduce without seeds. Growing a new plant from a stem cutting is an example of vegetative propagation, a type of asexual reproduction. The new plant is genetically identical to the parent.

Photoperiodism is the response of plants, especially in flowering, to the relative lengths of day and night. Some plants flower when days are long (long-day plants), some when days are short (short-day plants), and some are not affected (day-neutral plants). Plants detect this using a light-sensitive pigment called phytochrome.