GROUPS OF PLANTS. 3 



But with the Hght which has been thrown on the relationship 

 of the higher and lower groups of plants by the more recent 

 study of the lower forms the older terminology has been somewhat 

 modified. Thus, for example, w'e speak of the root and shoot, 

 with its leaves, as the vegetative organs of the higher plants, 

 and in describing the corresponding organs (where they exist) in 

 the lower plants, we either apply these terms directly, or indi- 

 rectly by saying that the latter are root-like, stem-like, etc. On 

 the other hand, we now speak of the sexual organs of the higher 

 plants as antheridia and oogonia (or archegonia) instead of 

 classifying them roughly as stamens and pistils, the latter names 

 being retained but with a different signification. 



Factors Influencing Growth. Plants have certain inherent 

 or inherited tendencies or characters which make up the inner 

 constitution, and this can not be modified by external agencies 

 except within more or less narrow limits. Depending upon this 

 character w^e find plants as different in kind as the apple tree 

 and pine growing under precisely the same conditions. In other 

 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 



