156 STUDY OF COMMON PLANTS. 



genus Pinus, and the different species of juniper, 

 the genus Juniperus. We have thus far studied 

 one or more representatives of each of the genera 

 Pinus, Juniperus, Picea, Tsuga, Thuja. 



5. Closely related genera constitute a family. The 



genera just named, with a number of others, make 

 up the Coniferse, or Pine family. 



6. Closely related families constitute higher groups, 



sometimes designated as orders, though the usage 

 is not uniform. Finally, orders (of flowering 

 plants) are grouped together in the great classes 

 gymnosperms, monocotyledons, and dicotyledons. 



The relationships here pointed out are those of descent. 

 It is believed that just as all individuals of a species are 

 descendants of a common ancestor, so all the species of a 

 genus and all the genera of a family have a common, 

 though remote origin. 



We shall have constant opportunity in our further study 

 of plants to become acquainted mth specific, generic, and 

 family characters. Their recognition is frequently at- 

 tended with some difficulty, and in all cases the exercise 

 of careful judgment is required. In fact botanical work 

 consists very largely in accumulating evidence by which 

 degrees of relationship are determined. 



SPECIAL STUDIES. 



I. Comparison of the cycle of development of gymno- 

 sperms with that of the vascular cryptogams. 



II. Anatomical characters of leaf and stem as a means 

 of determining relationship. 



