INTRODUCTION 3 



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 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 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 ele- 

 ments. 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 



