The Various Substances Made by Plants 115 



the same as that for cellulose and starch, but with n meaning 

 another figure; and they are formed no doubt from grape sugar, 

 (probably via the mucilaginous modification of cellulose men- 

 tioned in the preceding paragraph), to which they are readily 

 digested back by both plants and animals. 



Fruit-Jellies. These substances are familiar to all housekeepers 

 as the jelly which forms when fruits or vegetables are cooked 

 (e. g., grape jelly, orange marmalade, pumpkin preserves), though 

 it must be remembered that gelatine, from which the jellies of the 

 tea-table are made, is an animal product. In the living plant 

 they are solid, being insoluble in cold water; but they are dissolved 

 by hot water, which explains why they appear after cooking. 

 They represent, it is believed, another form of reserve food. Chem- 

 ically they are known as pectins, and they have also the same 

 general formula as starch (C 6 H 10 5 )n. They are formed without 

 doubt from grape sugar to which they are easily digested back. 



In reading this account of these various carbohydrates, two 

 questions will inevitably arise in the mind of the reader. First, 

 he will ask how it is possible that substances with properties so 

 different as those of starch, cellulose, gums and jellies can have 

 the same chemical composition. The answer is this, that on the 

 one hand the letter n in these formula represents without doubt 

 a different number in each case, and hence the composition is not 

 really identical, while on the other, even an identical formula 

 can be associated with very different properties, because the 

 properties depend not only on the elements present, but upon 

 the way these elements are arranged in the molecule; and they 

 can be arranged in very different ways. The differences between 

 grape sugar and fruit sugar are wholly of this latter kind. The 

 second question the reader will wish answered is this, why do 

 some plants store up their reserve food in the form of sugar, some 

 as starch, others as cellulose, and others as oil, soon to be men- 

 tioned. This question we cannot yet answer with certainty, but 

 probably the general explanation offered in Chapter III for the 



