SUCCULENT AND WATERY FOODS 249 



90 per cent, of water, and consequently have a specific gravity 

 of nearly one. It is said that " white turnips just float and 

 swedes just sink," but many individual specimens may be 

 found in which this rule is reversed. 



It was at one time supposed that the specific gravity of the 

 roots would afford a reliable indication of their nutritive value, 

 but it has been definitely established that this is not even 

 approximately true. The specific gravity of any individual 

 root apparently depends mainly upon the size and number 

 of the air cells it contains. Those which contain the most, i.e. 

 those which have the lowest specific gravity, will, under 

 similar conditions probably germinate most readily, and will, 

 therefore, be the least suitable for " keeping." 



The carbohydrates consist chiefly of pectocelluloses and 

 sugars. In sugar beet the latter compound is almost entirely 

 of the cane sugar type ; in others, e.g. swedes, mangels, etc., it 

 consists wholly or partially of glucose. The percentage of 

 sugar is smallest in the stalks, and increases from above down- 

 wards. Samples of roots should, therefore, always be radial 

 sections through the centre. The whole of the sugar occurs 

 in solution in the juice, and the larger part of it may be 

 extracted by pressure. 



The percentage of fat is so small as to be practically 

 negligible. The total nitrogenous matter may amount to 

 from i to i\ per cent., but it is practically all non-pro teid in 

 character. The nutrients of the roots are, therefore, almost 

 entirely carbohydrate. 



The sugars, being in solution, are of course completely 

 digestible. The pectocellulose, or pulp, is extremely soft and 

 easily hydrolysed. The whole of the non-cellulose portion 

 (p. 29), and probably also the larger part of the cellulose, is 

 digestible. In fact, the only indigestible portion of the root 

 appears to be the lignified fibre of the outer rind, and this 

 part is often not eaten by the animals. On reference to the 

 table of composition (p. 213), it will be seen that apart from 

 the amount of water they contain and this is too variable to 

 be truly characteristic turnips, swedes, mangels, and carrots 

 do not differ much in chemical composition as determined by 



