398 PHYSIOLOGY 
digestive organs comparable to these; but places of food making and food 
storage must be places where digestion is also particularly active. 
Misleading comparisons of the leaves to the stomach not rarely occur in primary 
books, which thus seek to " explain " the work of a leaf. When, as in one notable 
Instance, a leaf is compared to a kitchen, where the dilute " soups," coming up from 
the roots, are " boiled down "; later to a stomach, where the food is made ready; 
and finally to the lungs, by -which the dear little plant breathes, the child would have 
a truly appalling notion of a leaf were he not usually immune to such bad pedagogy, 
by reason of his ignorance of at least the stomach and lungs. 
Extra-cellular digestion. In plant as in animal, many foods must be 
digested before they can enter the cells at all, while others are digested 
as they lie in the cells. So one may distinguish, as to location, extra- 
cellular and intra-cellular digestion; but agents, processes, and results 
are essentially alike in both. In a fungus which merely pushes its way 
among the intercellular spaces of another plant, it is impossible to say 
whether any food is being digested or whether only what is already 
soluble and diffusible is being used. But when a fungus sends a branch, 
as a haustorium, through the cell wall (fig. 651), or when, as in certain 
wood-destroying fungi, the mycelium penetrates the walls freely in all 
directions, it is obvious that by some means the wall is actively dissolved 
at the point of contact. 
Chemical changes. The changes characteristic of digestion result 
in the cleaving of compounds into two or more simpler substances, with 
or without the taking up of water. In case water is incorporated the 
cleavage is called hydrolysis. 
Thus when cane sugar is digested: 
Ci 2 H 22 Oii + H 2 ^ C 6 Hi 2 O 6 
saccharose water glucose fructose 
Starch when digested takes up water, and four fifths of it breaks up into maltose 
units (Ci 2 H 22 On), the other fifth resisting full digestion for a long time. The mal- 
tose is further digested into two units of glucose, with assumption of another mole- 
cule of water. Other foods split up into simpler compounds without adding 
anything to their members. Thus sinigrin, a glucoside characteristic of the plants 
in the mustard family, cleaves thus: 
CioHi 8 NKS 2 Oio ^ C 8 H 6 CNS + C 6 H 12 O 6 + KHSO 4 
sinigrin allyl thiocyanate glucose potassium-hydrogen 
(mustard oil) sulfate 
The chemical changes of digestion represent only a few of the mul- 
titudinous reactions going on in the plant. The rate of these reactions, 
like all others, depends on temperature, concentration, etc., and espe- 
