Select the correct statement in reference to the pollen tube. Pollen tube exhibits :

Question

Accepted Answer

The correct option is 3: Chemotropic movement

The growth of the pollen tube is a classic and vital example of **chemotropism** in plants, which is a type of directional growth movement in response to a chemical stimulus. After pollination, the pollen grain germinates on the stigma and produces a pollen tube that must grow precisely down the style to reach the ovule inside the ovary for successful fertilisation. The ovule, or more specifically the synergid cells within the embryo sac, releases a gradient of specific chemical substances, known as chemoattractants (such as certain peptides or calcium ions). The pollen tube senses this chemical gradient and directs its tip growth towards the area of highest concentration, exhibiting a **positive chemotropic** response. This directional growth ensures the accurate delivery of the male gametes directly into the embryo sac, a critical step that overcomes the problem of immotile male gametes in flowering plants.

The incorrect option is 1: Chemotactic movement

**Chemotactic movement** (or chemotaxis) refers to the directional *locomotion* of a whole, motile organism or free-moving cell in response to a chemical gradient. This is typically observed in bacteria swimming toward nutrients, or in animal sperm or white blood cells moving toward attractants. The key difference is that chemotaxis involves *active movement* or *locomotion* of the entire cell mass. In contrast, the pollen tube exhibits directional *growth* (elongation of a tube structure), which is the defining characteristic of a tropism, not a taxis.

The incorrect option is 2: Chemonastic movement

**Chemonastic movement** (or chemonasty) is a non-directional movement of a plant part in response to a chemical stimulus. Nastic movements are not oriented with respect to the source of the stimulus. An example would be the opening and closing of a flower in response to atmospheric chemical changes or the rapid folding of tentacles in an insectivorous plant like *Drosera* when nitrogenous compounds are detected, where the direction of folding is predetermined, irrespective of where the chemical touched. Since the pollen tube's growth is specifically **directional**—it is oriented toward the source of the chemical (the ovule)—it cannot be classified as a nastic movement.

The incorrect option is 4: Thigmotropic movement

**Thigmotropic movement** (or thigmotropism) is the directional *growth* response of a plant part to a stimulus of touch or physical contact. The most common example is the coiling of climbing plant tendrils around a support structure. While the pollen tube does grow in contact with the style's tissue, its **directional guidance system** is primarily chemical, originating from the ovule, not a physical response to the touch of the surrounding cells. The growth of the pollen tube is fundamentally an attraction to a chemical gradient rather than a response to physical contact with the cell walls along its path.