362 PRACTICAL COURSE IN BOTANY 



ing degrees of irregularity, like the coast line on a map. 

 The relative positions of the different orders we have 

 been considering are represented by upright and diagonal 

 lines, the general course of which, as indicated by the 

 arrows, is intended to give an idea of the trend of evolu- 

 tionary progress in the particular group represented by each 

 line. No one of these lines is made to originate directly in 

 any other, because, with the possible exception of the mono- 

 cotyls, we have no authority for asserting that any such direct 

 connection exists between plants as we know them, but only 

 that certain types give evidence of descent from a common 

 ancestry. This lack of certainty is expressed by placing the 

 point of origin for any given line in more or less close proxim- 

 ity to the one which is supposed to be the nearest living 

 representative of the common ancestor. The line of ferns, 

 for instance, is depicted as originating in the region of the 

 bryophytes, somewhere in the neighborhood of the liverworts, 

 but the two lines nowhere come in contact, because there is 

 no evidence that any fern, living or fossil, is directly de- 

 scended from any particular kind of liverwort known to us. 

 With these explanations, the diagram shows, in a rough way, 

 the generally accepted view of plant relationships as based on 

 the evidence at present before us. But in questions of this 

 sort it is wise to keep in mind the blunt remark of a famous 

 old American statesman, that ''only fools and dead people 

 never change their opinions." 



Field Work 



1. If you live in the country, study the appearance of plants affected 

 with bhghts, smuts, rusts, and mildews, and learn to recognize the different 

 kinds of disease by their signs. Notice which kinds are most prevalent in 

 your neighborhood, and what plants are most affected by them. 



2. Notice the different kinds of mushrooms you find growing wild. 

 Observe the difference between those that grow on the ground and those 

 that grow on logs, stumps, and trees ; between those found in the woods 

 and those in open ground. Find out how those on the ground get their 

 nourishment. Uncover the mycelium, and notice the extent of its surface. 



