BIOLOGY Volume 2 - A Guide to General Biology - 2004

16. PLANT COORDINATION AND REGULATION

Like animals, plants require internal coordination of their Functions to ensure that GROWTH AND DEVELOPMENT proceed in an orderly manner and that the Organism as a whole responds appropriately to changes in its external environment. Unlike animals, however, plants lack a Nervous system; consequently, Intercellular Communication is purely hormonal, meaning they rely exclusively on chemical coordination factors. As a result, plants respond to stimuli more slowly, and their response is often limited to Changes in the growth rates of specific body parts. Growth, in turn, can result in the movement of a particular organ. In this chapter, we will first examine Plant Movements and then explore the various mechanisms that coordinate their functions.

16.1. Plant Movements

A defining characteristic of all plants is their inability to undergo locomotion, meaning they cannot move the entire organism from place to place¹. Nevertheless, plants are capable of movement in individual Organs, and these movements are determined by the plant's sensitivity to external stimuli. Movements caused by external factors are divided into two main categories: Tropisms and Taxes.

16.1. What is the primary reason why animals can move around, whereas plants cannot?

16.1.1. Tropisms

A tropism is a directional growth movement of a part of a sessile organism (a plant, fungus, or attached animal) induced and directed by an external signal. In plants, such movement is almost invariably driven by differential Cell growth. Tropisms are classified as positive or negative depending on whether they are directed toward or away from the stimulus. Plagiotropism is growth at an angle to the direction of gravity, with diatropism being a special case where this angle is exactly 90°. Some Examples of tropisms are given in Table 16.1.

Class="center">Table 16.1. Examples of tropisms

Stimulus	Type of tropism	Examples
Light	Phototropism	Positive phototropism is exhibited by shoots and coleoptiles. Negative phototropism is seen in certain roots, such as the adventitious roots of ivy-like vines
Gravity	Geotropism (gravitropism)	Negative geotropism is characteristic of shoots and coleoptiles. Positive geotropism is typical of primary roots. Lateral roots and branches exhibit plagiogeotropism, while rhizomes, stolons, and the leaves of dicots show diageotropism
Chemical factors	Chemotropism	Positive chemotropism is characteristic of the hyphae of certain Fungi (e.g., Mucor) as well as pollen tubes (in response to substances secreted at the ovule micropyle)
Water	Hydrotropism (a form of chemotropism)	Positive hydrotropism is observed in roots and pollen tubes
Contact with a solid object	Haptotropism (thigmotropism)	Positive haptotropism is displayed by tendrils (e.g., in pea leaves) and the glandular hairs on the upper surface of sundew leaves (a carnivorous plant)
Air (oxygen)	Aerotropism (a form of chemotropism)	Negative aerotropism is exhibited by pollen tubes

16.2. Add a fourth Column to Table 16.1 and indicate the adaptive advantage that each response provides to the plant.

Phototropism and geotropism are discussed in greater detail in Sections 16.2.1 and 16.2.2.

¹ This statement is not entirely accurate regarding species that produce stolons (runners) or rhizomes. The formation of new shoots away from the parent plant, followed by the death of the parent, results in the genetic individual shifting to a new Location. — Transl. note.

Last update: 06/08/2026

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