Cooks, whether told to use celery in heads or ounces, would act on guess-work just the same.

What are absolutely essential are two things--common sense and experience.
Again, practically, we must avoid giving too many ingredients.

Novices in the art of cooking are, of course, unable to distinguish between those vegetables that are absolutely essential and those added to give a slight extra flavour, but which make very little difference to the soup whether they are added or not.

We are often directed to add a few leaves of tarragon, or chervil, or a handful of sorrel.

Of course, in a large kitchen, presided over by a Francatelli, these are easily obtainable; but in ordinary private houses, and in most parts of the country, they are not only unobtainable but have never even been heard of at the greengrocer's shop.
In making soups, as a rule, the four vegetables essential are, onion, celery, carrot and turnip; and we place them in their order of merit.