24 GROWTH AND WORK OF PLANTS 



changed to grape sugar (or invert sugar, as it is sometimes called) 

 by adding a few drops of strong hydrochloric acid to a solution of 

 cane sugar and boiling for a minute or two. By neutralizing this 

 with sodium carbonate, Fehling's solution can be used to show 

 that it is grape sugar. If Fehling's solution is added to a solution 

 of cane sugar, and boiled, it will be inverted to grape sugar as the 

 red precipitate will show. Both kinds of sugar are sometimes 

 found in the same plant tissue. Both are apt to be present in the 

 branches of the sugar maple during the autumn, winter or spring. 



34. Proteins. Proteins have been mentioned as being pres- 

 ent in the aleurone layer of wheat. They also occur in the seeds 

 of other cereals. They are present in considerable quantity in 

 beans and other legumes. They form the principal food storage 

 in the thick leaves of the onion. Their presence can be deter- 

 mined by adding a small quantity of strong nitric acid to portions 

 of the tissue and then heating. A pale yellow color appears. If 

 a small quantity of ammonium hydrate is now added the color 

 is changed to orange. 



35. Experience shows that several food substances are 

 present in the same object studied. For example, starch, sugar 

 and proteins are present in the potato tuber, but starch is more 

 abundant than the other substances. 



36. Oil. Oil occurs in the tissues of various plants, but espe- 

 cially in certain seeds as cotton seed, flax seed, and in many nuts. 

 The fatty oils occur in the form of small translucent drops in 

 certain cells. The oil is obtained from seeds and nuts by sub- 

 jecting them to great pressure. Osmic acid blackens the oil and 

 its presence in tissue can be detected by the use of this reagent, or 

 by soaking pieces of the tissue in Flemming's solution which con- 

 tains osmic acid. Chloroform dissolves the oil from the tissue. 



37. Inulin. The food substance stored in the roots of certain 

 composite plants, like the Jerusalem artichoke (Helianthus tuber- 

 osus) and the dahlia, is in solution. If their tissues are immersed 

 for a long time in alcohol, inulin is precipitated into sphaero- 

 crystals. Many such crystals are sometimes present in a single 

 cell. They show peculiar radiate and concentric markings. 



