SEEDS AND SEEDLINGS 145 



or the watery sap of plants. But the insoluble substances be- 

 fore being transferred into the seedling are transformed into 

 soluble ones. This is due to the action of certain substances 

 known as enzymes or soluble ferments. An enzyme as found in 

 seeds is a substance secreted 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. Much has yet to be learned about the 

 nature, occurrence, and action of the enzymes. In most seeds 

 enzymes occur inside the cells along with the reserve mate- 

 rials, and so at suitable temperatures, in presence of moisture, 

 the digestion of the cell contents can take place everywhere 

 throughout the seed. The scutellum of the grains (Fig. 1 29, sc) 

 secretes, from its outer layer, which is in contact with the 

 endosperm, two kinds of enzymes, and rapidly digests the sur- 

 rounding endosperm. 



At very low temperatures, enzymes cannot carry on their 

 work. Each kind has a special temperature at which it is 

 most active ; for many kinds this ranges between 86 and 

 113 Fahrenheit (30 to 45 Centigrade). Evidently the low- 

 est temperature at which the enzyme of a given seed can act 

 must limit the temperature at which germination can go on l ; 

 and the temperature at which the growth of the very young 

 seedling is most rapid must be not far from the temperature 

 at which the enzyme of its seed is most effective. * 



Of the many kinds of enzymes known, two of the classes most 

 important in plant physiology are those known as diastases, 

 which change starch into sugar, and those known as trypsins, 

 which render insoluble proteins soluble. 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 10,000 

 times its own bulk of starch into sugar. 2 



1 See the table of germination temperatures, Sect. 130. 



2 On digestion and enzymes consult J. R. Green, Vegetable Physiology, 

 chap. xvi. P. Blakiston's Son and Co., Philadelphia. 



