Why did G. Mendel pollinate pea plants artificially?

Mendel decided to observe the phenomenon of interest to him – heredity – in peas. Peas are very convenient for studying heredity, it is easy to grow and the whole life cycle passes quickly. He is also prone to self-pollination, and without self-pollination, Mendel’s experiments would be impossible.
First of all, the trait whose inheritance is observed should clearly differ visually. The easiest way to take a sign, which is manifested in two ways. Mendel chose the color of the cotyledons. Pea seed cotyledons can be either green or yellow. Such manifestations of the trait are clearly distinguishable and clearly divide all seeds into two groups.
In addition, you need to be sure that the observed pattern of inheritance is a result of crossing plants with different manifestations of the selected trait, and not caused by some other circumstances.
Mendel grew two lines of peas, in one of which only green seeds appeared, and in the other – only yellow ones. Moreover, for many generations in these lines the pattern of inheritance has not changed. In such cases (when there is no variability in a number of generations) they say that a clean line is used.
Mendel did not know all the factors affecting heredity, so he made a non-standard logical move. He studied the results of crossbreeding between plants with cotyledons of the same color (in this case, the descendants are an exact copy of the parents). After that, he crossed plants with cotyledons of different colors (one has green, the other yellow), but under the same conditions. This gave him reason to argue that the differences that appear in the pattern of inheritance are caused by different phenotypes of the parents in these two crosses, and not by any other factor.

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