The human need for food is considered in Food Science, pp. 160-224. Precaution in production of vegetables is of great significance. Such food as green vegetables, being often eaten raw and being without covering to remove, such as fruit has, can carry disease from all sources. Fertilization of green gardens with waste products of living, as sewage, may propagate human disease and is to be avoided. Scrupulous cleanliness is essential with such foods. Even washing in boiled water vegetables to be eaten raw is advised if the purity of the water-supply is in any doubt.

Plants show their health and vegetables their quality readily upon observation. But skill in seeing comes only with looking and learning for what to look. Plants droop and die when not sound or cared for well. Vegetables wilt and decay when their vitality is waning. Such indications show the state of health of the food itself. The human disease germs a food may carry may have no apparent effect upon the food itself; the danger is to those who eat food so laden. Precaution against dust everywhere, flies, insects, and any form of contact with illness or waste-products is too little practiced anywhere.

Vegetables differ widely in coarseness and fineness according to the care exercised in their production. This is notably so in lettuce. Superior production should be practiced. Such difference in food-quality is not to be confused with natural difference in degrees of fineness, as in cabbage and cauliflower, that are otherwise so much alike. Both these are desirable. Cabbage is coarse, yet it can be chopped and so prepared as to be a delicate food. This precaution should be taken. Cabbage is more digestible so.

Digestibility of food as well as its composition determines its nourishing power. About 85% of vegetables is digestible.