106 THE STRUCTURE AND WORK OF THE STEM 



Experiment showing the osmosis of 

 sugar (right-hand tube) and non- 

 osmosis of starch (left-hand tube) . 



end of both tubes was placed in a glass dish under water. After 

 twenty-four hours, the water in the dish was tested for starch, 



and then for sugar. We find that 

 only the sugar, which has been 

 dissolved by the water, can pass 

 through the membrane. 



Digestion. — As we shall see 

 later, the food for a plant is manu- 

 factured in the leaves or in stems, 

 etc., wherever green coloring mat- 

 ter is found. Much of this food 

 is in the form of starch. But 

 starch, being insoluble, cannot be 

 passed from cell to cell in a plant. 

 It must be changed to a soluble 

 form. This is accomplished by 

 the process of digestion. We 

 have already seen that starch was 

 changed to grape sugar in the corn by the action of a substance (a 

 digestive ferment) called diastase. This process of digestion seem- 

 ingly may take place in all living parts of the plant, although most 

 of it is done in the leaves. In the bodies of all animals, including 

 man, starchy foods are changed in a similar manner, but by other 

 digestive ferments, into soluble grape sugar. (See experiment, 

 page 72.) 



The food material may be passed in a soluble form until it comes 

 to a place where food storage is to take place, then it can be trans- 

 formed to an insoluble form (starch, for example) ; later, when 

 needed by the plant in growth, it may again be transformed and sent 

 in a soluble form through the stem to the place where it will be used. 

 Building of Proteids. — Another very important food substance 

 stored in the stem is proteid. Of the building of proteid, little is 

 known. We know it is an extremely complex chemical substance 

 which is made in plants from compounds containing nitrogen, the 

 nitrates and compounds of ammonia received through the roots 

 from the organic matter contained in the soil, combined with sugar 

 or starches in the body of the plant. 



Some forms of proteid substance are soluble and others insoluble 



