As a seed germinates and seedling grow-up into young plant the size of the cotyledons gradually become smaller because???

Because the seedling consumes the nutrients that the cotyledon has as it grows.

Explanation:

Cotyledone is a part of the seed embryo. It is a structure that stores nutrients that will sustain the seedling, from germination until the moment it develops the first green leaf. As you may already know, plants feed through photosynthesis, but seedlings develop this ability little by little. While the seedlings cannot efficiently photosynthesize, they will consume the nutrients from the cotyledon. As these nutrients are consumed, the cotyledone gets smaller and smaller, until it dries completely and comes off the stem.

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