. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE SOURCE AND USES OF FOOD 139 directly, is accomplished by energy released in the decomposition of these foods or substances made from them. The green leaves, therefore, are the organs where the sun's energy is transformed so that it can be stored for use afterward where and when it is needed. They are the organs where the materials of the inorganic world are assembled in such a form that they can be used in the construction of the living body. How energy is obta
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE SOURCE AND USES OF FOOD 139 directly, is accomplished by energy released in the decomposition of these foods or substances made from them. The green leaves, therefore, are the organs where the sun's energy is transformed so that it can be stored for use afterward where and when it is needed. They are the organs where the materials of the inorganic world are assembled in such a form that they can be used in the construction of the living body. How energy is obtained in the decomposition of food will fippear from a specific example: When a definite amount of sugar or starch is made within the leaf from carbon dioxide and water a certain amount of sun's energy is employed and transformed from the active or kinetic to the passive or poten- tial state. This energy is not destroyed or lost; it is simply changed from one condition to another. To illustrate: a cer- tain amount of energy is required to lift a brick from floor to table. The energy spent in doing this now rests quiescent in the brick in virtue of its posi- tion above the floor, and reap- pears as active energy when the brick falls. So when sugar or starch is transformed into carbon dioxide and water from which it was made the energy from the sun employed in com- pounding it is set free (com- parable to the falling of the brick) and a part of it can then be used by the plant for other purposes, as in the construction of proteids from carbohydrate, etc., the synthesis of protoplasm, the overcoming of resistance, and in other ways. The sun's energy made potential in sugar and the like is, by the translocation of these substances, distributed to all parts of the body where every living cell can make use of it, and where, if not immediately wanted, it can be stored for use at some future Fig. 69.—Cross section of a portion of the blade of a leaf, showing upper epidermis at
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