THE PLANT FORMATION 



229 



neighbors. Occasionally a vigorous weed invades and by strong 

 competition conquers a small area. This frequently hajjpens 

 when the original plants are destroyed over a small space, thus 

 permitting some annual to enter and increase rapidly. Hordeum 

 and Lepidium families are especially apt to appear in this fashion 

 in grassland. 



Experiment 60. The structure of a formation. Examine a prairie, 

 meadow, or forest for families and communities. Make a chart of 

 each and note the differences between them. 



Run a line transect through the formation from east to west, and 

 north to south by pacing. Note the families, communities, societies, 

 and consocies encountered. 



243. Layers. Forests and thickets show a more or less definite 

 arrangement of the plants l3elow the facies into layers. There is 



Fig. 89. The Erythruniwn layer in the early spi-ing ai>pect of the oak- 

 hickory forest at Lineoln. 



also a suggestion of the latter in many grassland formations, while 

 some thicket-like formations of tall herbs show much the same 

 conditions as ordinary thickets. Layers are the result of the 

 habit of growth of the various species, and this is largely a response 



