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. 1 



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, com 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, 



