INTRODUCTION 3 



occur in cells hidden from the light. One group of these leuco- 

 plasts forms the starch that appears in underground storage 

 organs as in the potato (Fig. 2, ). Usually spaces or vacuoles, 

 that appear as cavities also occur in the granular cytoplasm 

 (Fig. 2, A, v). In reality they are filled with watery solutions of 

 various substances, the so-called cell sap. Many other structures 

 (Fig. 2, D) appear in the protoplasm to which attention will be 

 called later. It is well to remember that these visible structures 

 do not represent the real composition of the protoplasm. A 

 variety of units beyond the range of visibility, grow, multiply, 

 and build up the various structures which we recognize as con- 

 stituting the protoplasm. The cell walls make up the body of 

 the plant and give stability to its various organs but the living 

 part of the cell is the protoplasm. This substance reaches 

 through delicate pores in the cell wall to the protoplasm of 

 adjacent cells so that all the living substance of the entire plant 

 body is in contact and forms one united mass. 



2. The Nature of the Living Substance of the Plant. The pro- 

 toplasm possesses most remarkable powers. It can absorb vari- 

 ous fluids and gases, decompose them into simple elements, re- 

 unite them into foods or discharge from the cell such substances 

 as are not required. Furthermore the protoplasm effects those 

 changes termed growth by transforming the foods into the sub- 

 stances that compose the cell walls and other parts of the cell 

 as the protoplasm itself. How are these changes brought about? 

 Every substance is composed 'of elements. Water consists of 

 two elements, hydrogen and oxygen. The elements that compose 

 any substance are held together with great energy, owing to their 

 mutual attraction for one another. When two different sub- 

 stances are brought together, it may happen that the attraction 

 of certain elements of one substance is greater for one or more 

 of the elements of the other substance than for its own elements. 

 The result is, that the elements will be torn away from their 

 respective substances and united into new combinations. We 

 say that a decomposition and a re-combination has been effected, 

 or, that a chemical reaction has occurred. We see an illustration 

 of these chemical changes when iron-ore is heated with charcoal. 



