166 INTRODUCTION TO BOTANY 
by the plant for the purpose of digesting or rendering soluble 
such plant foods as require digestive action before they can be 
absorbed by the tissues of the young seedling. 
The most familiar case of action of enzymes on a large 
scale is the malting of barley, in which the starch of the grain 
is converted into a sugar by diastase. It is said that diastase can 
change ten thousand times its own bulk of starch into sugar. 
158. Propagation due to seeds. Annual plants evidently owe 
their continued existence to the growth of new crops from the 
seed. If every grain of Indian corn in the world were to be 
consumed during some winter, corn plants could not again be 
grown. Many bulb- and tuber-bearing perennials could con- 
tinue to propagate their kind for an indefinitely long period 
without seeds. Some herbs, such as the common field sorrel, 
and many shrubs and trees, such as rosebushes, black locusts, 
and silver-leaved poplars, reproduce themselves abundantly 
by buds formed on the roots, but most trees, and especially 
nearly all conifers, such as the pines, spruces, and firs, are 
usually propagated only by seeds. 
People in general hardly recognize the wonderful capacity 
of seeds for carrying on plant life under extremely adverse con- 
ditions. Most flowering plants soon die if they are entirely 
deprived of water for a few days; darkness is fatal to them 
and very low temperatures kill many kinds of plants in a few 
minutes; but the seed may be kept for months or years with- 
out water, in absolute darkness and at the lowest temperature 
ever encountered on the earth’s surface, and yet remain ready 
to grow as soon as it is exposed to conditions favorable to 
germination. 
159. Need of seed dispersal. The successive crops of farm 
and garden annuals are secured by careful seed planting in 
prepared soil. The seeds of wild plants are also sown, on a 
still more extensive scale, by natural agencies. In any coun- 
try the relative numbers of most kinds of wild seed plants 
1 On digestion and enzymes consult J. R. Green, An Introduction to Vege- 
table Physiology, Chapter XVI. P. Blakiston’s Son & Co., Philadelphia. 
