in Photomorphogenesis by

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Phytochrome A (Phy A), present only in the angiosperms which is responsible for the germination and de etiolation of seedlings. In the darkness the phytochrome A accumulates at high levels and the action of Phy A dependent upon the light.

Whereas the Phytochrome B (Phy B), is the photoreceptor which is involved in the shade detection and avoidance. There by increases the elongation rate of the stem when they become shaded by competitors. It is also responsible for the day length in the flowering.