INTRODUCTION 3 



organs as in the potato (Fig. 2, E). 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 va- 

 rious 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 

 may not represent the real composition of the protoplasm. It 

 has been supposed that a variety of units beyond the range of 

 visibility, grow, multiply, and build up the various structures 

 which we recognize as constituting 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 substances 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. 

 The ore is composed of the elements iron and oxygen and the 

 charcoal consists of carbon. In the presence of heat the carbon 



