2 PLANT GELATIONS. 



associations are as numerous as are the conditions of living, 

 and it may be said that each association has its own special 

 regulations, which admit certain plants and exclude others. 

 The study of plant associations, to determine their conditions 

 of living, is one of the chief purposes of botanical field work. 



3. Plants as living things. — Before engaging in a study 

 of associations, however, one must discover in a general way 

 how the individual plant lives, for the plant covering of the 

 earth's surface is a living one, and plants must always be 

 thought of as living and at work. They are as much alive 

 as are animals, and so far as mere living is concerned they 

 live in much the same way. Nor must it be supposed that 

 animals move and plants do not, for while more animals than 

 plants have the power of moving from place to place, some 

 plants have this power, and those that do not can move cer- 

 tain parts. The more we know of living things the more is 

 it evident that life processes are alike in them all, whether 

 plants or animals. In fact, there are some living things 

 about which we are uncertain whether to regard them as 

 plants or animals. 



4. The plant body ^— Every plant has a body, which may 

 be alike throughout or may be made up of a number of 

 different parts. When the green thready plants (algce), so 

 common in fresh water, are examined, the body looks like 

 a simple thread, without any special parts ; but the body of 

 a lily is made up of such dissimilar parts as root, stem, 

 leaf, and flower (see Figs. 75, 144, 155, 174). The plant 

 without these special parts is said to be simple, the plant 

 with them is called complex. The simple plant lives in 

 the same way and does the same kind of work, so far as 

 living is concerned, as does the complex plant. The differ- 

 ence is that in the case of the simple plant its whole body 

 does every kind of work ; while in the complex plant 

 different kinds of work are done by different regions of the 

 body, and these regions come to look unlike when differ- 

 ent shapes are better suited to different work, as in the 



