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: aniels, those living one year or less; b/en- 
nials, those living two years; perenuicls, those living more 
than two years. The boundaries between these classes are 
not always definite. For example, winter wheat is an annual, 
