Plants live. They grow from seeds. They develop the constitution of their plant family. Their developing is called maturing. They blossom, bear fruit, and produce seeds. This process repeated season after season is known as reproduction. Nature's method of continuing the life by vegetation is by physically renewing thus its products. Plant life gives definitely the processes of plant-living.

The readiness of plants for food-use and for reproduction of their kind is not usually the same, because in forming the seed the plant changes itself. The seed itself may be suitable food. When the seed is a human food the rest of the plant usually is not, as bean-pods. Cucumbers gone to seed are not good food, nor are potatoes raised for seed. When other parts than the seed are used for food, these are usually desirable when young or when just full-grown. Cellulose in young plants is tender, later woody. Green vegetables are therefore better young. Starch increases with maturity. Sugar when present does, too. Foods valued for these constituents are of course desirable only when these are produced in them.

Living substances in the main form human foods. Usually anything in food not derived from something that lives itself is not human food. Often such substances when introduced into food are not included in order to nourish the body, but to keep the food from such deterioration as would make its use impossible. It is only commerce overkeeping food and industry using inferior food that introduce non-food materials extensively into human food. Some condiments are of other than direct living origin. Common salt is, and is necessary to life.

Experience in living has taught humanity in which stage of development each plant is best as human food. This agelong habit is followed in choosing vegetable foods.