EXTERNAL CHARACTERISTICS 217 



is desirable to consider how certain forms of structure aid 

 the plant in living where we find it and necessarily restrict 

 it to certain kinds of surroundings. In so considering 

 plants, we are studying them as living things, and it is for 

 this reason that we have been considering leaves as related 

 to their environment ; we have not been tr)dng to explain 



Fig. 78. — Wild spikenard {Aralia racemosa) , a shade plant. This kind of leafage 

 is common in the woods. Such large, thin leaves are able to do their work in 

 dim light. 



why some leaves are large and some are small. It would 

 be quite wrong for you to get the impression that botanists 

 know the causes which produce the structures whose ad- 

 vantages or disadvantages they discuss. What seems to 

 be the cause may be only one cause among many, and this 

 whole matter of why plant structures are as they are is 

 not to be explained by simply noting a few external con- 



