ADJUSTMENT TO TEMPERATURE 103 



is brought about by the cotyledons, which likewise absorb the 

 digested material. When a large amount of food is stored in the 

 cotyledons, the latter usually remain in the ground, or, if carried 

 above it, they rarely become functional leaves. When the food 

 is packed about the cotyledons, they act as the first leaves of 

 the seedlings, except in the grasses. Here the single seed-leaf 

 is transformed into a special organ of absorption, the scutellum. 

 The digestion of stored material is carried on chiefly by the coty- 

 ledons, though the cells of the endosperm also play a part when 

 the latter is present. The conversion of starch into sugar is 

 effected by diastase, largely secreted by the cotyledons. The 

 latter often produce other enzymes as well, though these do 

 not always seem necessary to the removal of proteids. The foods 

 digested by the cotyledon are translocated to all parts of the 

 embryo, and vigorous respiratory action is set up to secure the 

 energy necessary for assimilation and growth. Such respiration 

 is regularly aerobic, though under abnormal conditions it may for 

 a time be anaerobic. Digestion continues until the food mate- 

 rial has been removed from the endosperm or the cotyledons, by 

 which time the seedling is provided with roots and leaves and 

 is again able to obtain its food and energy directly from photo- 

 synthesis. Respiration, on the other hand, is a continuous process. 

 Once actively begun in the embryo, it continues throughout the 

 life of the plant, disappearing only when the latter passes into 

 the seed stage or other resting condition. 



Experiment 33. Digestion and respiration in seeds. Germinate seeds 

 of the bean, sunflower, and Indian corn in a moist cliaml)er or a ger- 

 minator. As soon as the radicles appear, cut a median section of each 

 seed to show the relation of the various parts. Note especially the 

 condition of the reserve food as compared with that of the food in the 

 dry seeds. Remove a few of the corn embryos, and place them upon 

 moist potato-starch in a moist chamber. After some time, note the 

 action upon the starch grains. 



Place a number of peas in a bottle, cover them with water, and close 

 the bottle with a rubber stopper containing two holes. Insert a ther- 

 mometer in one of the latter, lowering the bulb to the water. Bend a 

 piece of glass tubing so that one end will fit in the second hole of the 

 stopper without reaching the water below, and pass the other end into 

 a stoppered bottle containing a 10% solution of lime-water or of barium 

 hydrate. Explain the precipitate that is formed. Compare the tem- 

 peratures of the germinating peas with those of the air outside. 



