296 TEXTBOOK OF BOTANY 



sidered fairly well settled ; although it is true that many new 

 species, even of seed plants, are described each year. 



299. Families of Angiosperms. — Something was said in 

 Chapter XVII about the reasons why flowers are helpful in 

 determining the relationships of angiosperms. For the most 

 part, it is the structure of their flowers and fruits that is used 

 in arranging angiosperms in orders, families, and genera; 

 species are distinguished from one another about as much by 

 the peculiarities of their vegetative parts (stems, roots, and 

 leaves") as by those of their flowers and fruits. As we already 

 know, angiosperms are divided first of all into the two great 

 groups of monocotyledons and dicotyledons. The mono- 

 cotyledons include eleven orders, which are divided into 

 forty-five families ; these in turn include a total of about 

 24,000 species. In the group of dicotyledons there are forty 

 orders, 240 families, and about 109,000 species. In the 

 present chapter only a few of the families of monocotyledons 

 and dicotyledons will be mentioned. These families are 

 selected either because of the large number of species that 

 belong to them, or because they include some of the plants 

 that are of greatest practical value to us. 



Monocotyledons 



300. The Grass Family. — This family contains about 

 4000 species, among which are the Indian com and the other 

 cereal grains. The grasses have small, simple flowers with 

 chaffy bracts, all much like the flowers of the com excepting 

 that thejr are usually perfect. The fruits, mth the exception 

 of a few that are nut-like or berry-like, are kernels, similar 

 to those of com and wheat. Each kernel contains a single 

 seed ; the embryo lies at one side of the seed, surrounded 

 by the seed leaf, and the endosperm makes up the greater 

 part of the seed. The grasses have jointed, commonly 

 hollow, stems, and alternate sheathing leaves arranged in 

 two vertical rows. With the exception of the bamboos, they 



