44 INTRODUCTION TO BOTANY 



if tested with iodine for starch. 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 

 moistened with nitric acid (the 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 and roots (fig. 30) often contain large 

 quantities of stored food and are thus useful in tiding the 

 plant over that period of the year when no food can be made. 

 In the same way they are of service in storing water, as has 

 already been shown (sect. 21). There are many shade plants, 

 such as trilliums, dogtooth violets, wild ginger, May apple, 

 and others, which leaf and flower early in the spring and do 

 a large part of the storing of food for the next season in their 

 rootstocks, tubers, or bulbs, before the trees under which they 

 grow are in such full leaf as to shut out the abundant light 

 necessary for photosynthesis. 



Fleshy leaves often contain much stored food, as in the famil- 

 iar century plant, which, after storing food for fifteen years or 

 more, may use this food in producing an immense flowering 

 stalk and many flowers and seeds. By the end of the flowering 

 season the leaves, in the case of century plants that were tested, 

 had lost more than 90 per cent of their weight. This flowering 

 stalk may reach a height of over 33 feet and a weight of some 

 500 pounds. Its average growth in height during the month 

 of most rapid elongation has been found to be about 5^ inches 

 a day. Not only the plant food but also nearly all the water 

 for this rapid growth is furnished by the leaves. 



44. Relation of food and water storage to duration of life. 

 It is usual to divide plants, according to their duration of life, 

 into three classes: annuals, those living one year or less; bien- 

 nials, those living two years ; perennials, those living more 

 than two years. The boundaries between these classes are 

 not always definite. For example, winter wheat is an annual, 



