LIVING THINGS 221 



187. Plant and Animal Forms. While plants are less fixed 

 as to form than animals, there are certain groups into which 

 they are naturally divided on account of their structure or 

 length of life. It is customary to group plants as annuals, 

 which live but a single season, and perennials which may live 

 for many seasons. The perennials are further divided into 

 the herbaceous species which die down to the ground at the 

 approach of cold or dry seasons, and the woody species which 

 do not. The latter are further divided into the trees with a 

 single woody stem, shrubs with several stems, and the lianes 

 or vines whose stems, though woody, are much too weak to hold 

 themselves erect and therefore climb on other plants. The 

 animals, instead of being grouped according to length of life, 

 as are the plants, are more frequently grouped into Verte- 

 brates in which there is a spinal column, and Invertebrates in 

 which this is lacking. The vertebrates contain the highest 

 types such as the fish, amphibians (frogs, etc.), reptiks, birds, 

 and mammals, but are greatly outnumbered by the inverte- 

 brates. The vertebrates never have more than four append- 

 ages for locomotion, and the highest division of them, the 

 mammals, nourish their young with milk. The most note- 

 worthy groups of invertebrates are the Mollusca, containing 

 the snails, clams, oysters, and other " shell fish," and the 

 Arthropoda which include the crabs, lobsters, spiders, and 

 insects. All in this latter group have more or less distinctly 

 jointed bodies, and from six to ten or more appendages for 

 locomotion. 



Practical Exercises 



1. Strip a piece of fresh epidermis from an onion bulb, mount in a drop 

 of water on a slip of glass and examine with a simple lens. The cells may 

 be easily seen. (If a compound microscope is available, it will show the 

 cell wall, cytoplasm and nucleus.) 



2. Mount any of the green growths (algse) found floating in ponds as 

 directed in exercise 1 and examine. Note the colored bodies which 

 function in food making. 



