128 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



presence of substances called enzymes, which act as catalyzing agents, 

 thus hastening by their presence the performance of such functions. 

 (See pages 279-280.) Enzymes are manufactured in certain cells 

 and it is believed that every cell, even an egg cell, contains enzymes 

 which are capable of digesting food substances, as well as those which 

 aid in oxidation within the cell. 



In plants, enzymes seem to be made in almost any cell that is active 

 and these enzymes usually have a reversible action. For example, 

 certain insoluble foods may be broken down or hydrolyzed in the 

 cells of the leaf, so that they are soluble, then they will pass in that 

 condition to the stem, the roots, or the fruit, where a reverse action 

 takes place and the food is stored in an insoluble condition. In 

 animals, the hydrolyzing enzymes which make digestion possible are 

 usually formed by groups of cells forming glands. 



Structure of Protoplasm 



This living material, known as protoplasm, has been called by the 

 biologist Huxley "the physical basis of life." It is this stuff that is 



always present in things 

 that are ahve. In our 

 present state of knowl- 

 edge we may liken it to 

 the albumen or white 

 of egg, a nearly colorless 

 and translucent sub- 

 stance, like soft jelly. 

 This substance seen un- 

 der the compound microscope has many granules floating in it. 

 It is more or less elastic, although in some cases it will flow like 

 a dense liquid. Seen under a high magnification it may be almost 

 homogeneous in structure or may appear foamy or spongelike, or 

 even fibrillar in appearance. A study of living cells shows that it 

 is obviously quite different in structure at different times and in 

 different animals and plants. 



^^C^. 



gretntjtlav 

 "structure 



Stritctvtre 



ctlveolecr 

 structure. 



Three states of protoplasm. 2 and 3 have much 

 higher magnification than 1. 



Protoplasm and the Cell 



Although cells were first described in 1665 by Robert Hooke, it 

 was not until the nineteenth century that the cell theory came into 

 the spotlight. The knowledge that all organisms, plant and animal, 

 are composed of fimdamentally identical protoplasmic units, or cells, 



