140 STUDY OF COMMON PLANTS. 



to begin with, on account of the simplicity and distinct- 

 ness of its floral structures. Many of the other genera 

 are likely to prove rather troublesome until the student 

 has had some experience. 



After careful comparison of as many different kinds of 

 grasses as practicable, summarize your observations in a 

 general account of the characters of the Graminese. 



This family includes some four thousand species and is 

 of great economical importance, since it furnishes, directly 

 or indirectly, by far the larger part of the food of the human 

 race. Botanically it presents many points of interest. 

 While there are many species of grasses within the tropics, 

 they form a characteristic "sod" only in the cooler % parts 

 of the world. Some depart so widely from the habits of 

 those we have studied as to be properly reckoned among 

 climbing plants. Although giving evidence of very con- 

 siderable modification, the flowers are, with few exceptions, 

 destitute of odor and attractive colors, and are either self- 

 fertilized or depend for fertilization on the agency of the 

 wind. The seeds are disseminated in a variety of ways, 

 some passing undigested through the alimentary canal of 

 herbivorous animals, others, as Cenchrus, bearing hooked 

 or spiny appendages, and still others, as Stipa, provided 

 with a twisting awn that attaches itself to the coats of 

 animals or buries the grain in the earth. In Tripsacum 

 the joints of the spike break apart and are often floated 

 away by water, while species of Panicum and Eragrostis 

 are blown about by the wind as " tumble-weeds." The 

 cultivation of the most important grains is prehistoric and 

 their origin uncertain. 1 



1 Cf. De Candolle, Origin of Cultivated Plants, p. 354 et. seq. ; Hackel, 

 The True Grasses ; Beal, Grasses of North America. 



