Explanation:

When there is an equal proportion of auxin and cytokinin in the medium for plant tissue growth, it promotes callus formation. Callus formation refers to the development of a mass of undifferentiated cells that can give rise to various types of tissues.

Reason:
The equal proportion of auxin and cytokinin in the medium creates a balance of growth regulators that leads to callus formation. This balance is known as the auxin-cytokinin ratio. Both auxin and cytokinin have different effects on plant growth and development, and their ratio determines the outcome.

Auxin:
Auxin is a plant hormone that promotes root formation. It stimulates cell elongation and differentiation, which are important for root development. When auxin is present in higher concentrations compared to cytokinin, it favors root formation.

Cytokinin:
Cytokinin is a plant hormone that promotes shoot formation. It is responsible for cell division and growth, leading to shoot development. When cytokinin is present in higher concentrations compared to auxin, it favors shoot formation.

Equal Proportion of Auxin and Cytokinin:
When there is an equal proportion of auxin and cytokinin, their effects balance each other out. This balance stimulates cell division and growth, but without a specific bias towards root or shoot formation. As a result, undifferentiated cells start proliferating and form a mass of cells called callus.

Callus Formation:
Callus formation is the first step in plant tissue culture, which is used for plant propagation and genetic engineering. The callus can be induced to differentiate into various types of tissues, such as roots, shoots, or somatic embryos, depending on the subsequent culture conditions and growth regulators used.

In summary, when there is an equal proportion of auxin and cytokinin in the medium for plant tissue growth, it promotes callus formation. This is because the balance of growth regulators leads to the proliferation of undifferentiated cells, which can later be induced to differentiate into specific types of tissues.