CELL-CONTENTS AND FORMS OF CELLS. 247 



words, the character of the structure is determined in the main 

 by the nature of the organism. It is true that an apple tree may 

 grow better in one locality than another, but it is still an apple 

 tree whether it be dwarfed or attain to the full measure of its 

 growth. These slight changes in the character are known as 

 accidental variations. Frequently they are the result of tempo- 

 rary conditions and are not repeated in the succeeding genera- 

 tion. On the other hand, if the special conditions remain these 

 individual variations may be repeated in generation after gen- 

 eration and finally become permanent characters. 



The gradual change in the structure and nature of organisms 

 which takes place through long periods of time is spoken of as 

 EVOLUTION. In some cases specific changes in the characters of 

 plants arise rather suddenly without any known cause, and such 

 changes are spoken of as saltations or MUTATIONS. 



The factors essential for growth in all cases are food, water, 

 and a certain temperature. Among the food elements we may 

 mention as of chief importance, carbon, hydrogen, oxygen, and 

 nitrogen. Some of the other elements are also essential to most 

 plants, although they occur in relatively small proportion in the 

 plant, as potassium, magnesium, phosphorus, sulphur, iron, and 

 calcium. The latter element does not seem to be necessary to 

 the normal development of some of the Fungi and certain Algae. 



Water permeates all parts of the plant, and when the cells 

 are in the normal turgescent state it contains more than half 

 its weight of water. When the supply of water falls below the 

 normal the plants begin to droop and finally die. The need of 

 plants varies greatly in this particular ; some are aquatic in their 

 habits and live wholly in the water ; others can live only on the 

 land ; and still others are adapted to desert regions. 



The degree of temperature necessary for growth varies within 

 certain limits for each kind of plant, but, as is stated by Pfeffer, 

 the greatest extremes are shown by Fungi, Bacteria, and the 

 lower Algae. Generally speaking, the most favorable temperature 

 for growth is between 24 and 34 C. 



Besides the factors enumerated there are other factors which 

 influence growth. They include light, gravity, mechanical 

 agencies, etc., and are sometimes spoken of as external stimuli. 



