STOKAGE IN STEMS AND LEAVES 



77 



69. Storage of food. Aerial stems contain plant food, often 

 in great quantities. In the trunks of trees this food is present 

 in various forms, as starch, 

 sugar, oil, and proteins. Many 

 kinds of sapwood turn deep blue 

 or black if tested with iodine for 

 starch in the autumn. During 

 the winter much of this starch 

 is often converted into sugar 

 or oil. The presence of proteins 

 in wood is so general that the 

 cheaper grades of white paper, 

 largely made of wood pulp, at 

 once turn yellow on being mois- 

 tened with nitric acid (protein 

 test). When thus tested, paper 

 made wholly of cotton, or of 

 linen, rags, shows little change. 

 The plant food stored in wood 

 "is most abundant in the younger 

 portions (sapwood), and above 

 all in the cambium layer. 



Underground stems often con- 

 tain large quantities of stored 

 food, and are thus useful in 

 tiding over the period of the 

 year when no food can be made, 

 just as they have already (Sect. 

 66) been shown to be of serv- 

 ice in storing water. There are 

 many shade plants such as 

 trilliums, dogtooth violets (Fig. 

 66), wild ginger (Fig. 43), May 

 apple (Fig. 59), and others 



FIG. 63. The century plant of the 



preceding figure as it appeared 



nearly two months later 



The leaves have given up their stored 



food to the flowers and flower stalk 



and are now withered and valueless. 



Photograph by G. D. Fuller 



which leaf and blossom early in the spring and do a large part 

 of the storing of food for the next season in their rootstocks, 



