STORAGE IX STE.MS AXD LEAVES 



77 





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

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



in various forms, — as starch, 



sugar, oil, and proteins. Alany 

 kinds of sapwood turn deep blue 

 or black if tested with iodine for 

 starch in the autumn. During 

 the winter much of this starcli 

 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 ^l■ith 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 abo^-e 

 all in the cambium laj'er. 



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), 3Iay 

 apple (Fig. 59), and others — 

 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. 



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 



