356 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
Again, though a certain diet may contain a sufficient amount of 
absorbable proteid, fat, and carbohydrate to maintain health it will not 
necessarily do so. It is known that the presence of chemical bodies 
known to chemists as aromatics is essential to health — such are found 
readily in beef and other meats. Indeed, it is entirely to them that 
beef-tea and meat extracts owe their properties. I merely mention 
this fact as a warning that a purely vegetable diet is not best fitted to 
preserve health. 
The term energy, in speaking of foods, is used in its mechanical 
sense. This energy is convertible into heat in the body, and by means 
of it the body can perform work. A simple illustration may be given : 
During its growth, a tree collects its energy from the sun heat. It is 
cut down and burnt. In the burning its energy is reconverted into 
heat ; this heat may be used to generate steam, which can do work in 
the shape of driving a piston. Instead of a tree let us take a potato ; 
it too collects energy from the sun heat, it is eaten and burnt up 
in the body, giving off heat which is used to produce the energy neces- 
sary to enable that body to do work. 
The unit of energy as applied to food is called the big calorie and 
represents the number of litres of water which I gramme of the tested 
food when burnt will raise i° C. in temperature. 
It is necessary briefly to mention this point as all calculations on 
food values are based on it. 
The energy values of the chief constituents of food are : 
Proteid . . . . 4-1 C. 
Carbohydrate . . . 4-1 C. 
Fat 9-3 C. 
The proteids and carbohydrates are quicker in action than the fats, 
but have not their staying power, so to speak. 
In deciding which are the best vegetables to grow for food supply, 
we must consider the following points : 
(1) Their food value as expressed in calories. 
(2) Their digestibility and absorbability in the human body. 
(3) Their economic value, gauged by their tests. 
(a) Expense of seed, growing, and harvesting. 
(b) Proportion of land to size of crop. 
(c) Time required for crop to reach maturity. 
(d) Plant food in soil used up. 
In allotment work we may consider (a) and (d) to be fairly constant, 
but (b) and (c) must be constantly kept in mind ; e.g. it is obvious that, 
if an acre of land will provide 6 tons of potatos or 1 \ tons of peas, the 
potatos are the best crop to grow, even though the food value of peas 
is greater than that of potatos. 
In offering you the following list I have calculated the units of value 
whilst keeping all these points in view. Also, I have only estimated 
for the absorbable part of the food, and, in the case of leguminous 
