140 GENERAL BOTANY 



are numerous varieties which are considered to belong 

 to one natural species, because their differences have 

 arisen only in cultivation. All the many kinds of 

 Paddy for instance belong to one species ORYZA SATIVA, 

 Linn., all our garden Crotons to CODI^UM VARIE- 

 GATUM, Blume. Well-defined varieties occur also wild 

 in a few species. 



When we have collected and examined a large 

 number of genera, we shall easily see that just 

 as species can be grouped into genera, so genera 

 fall naturally into more or less well-defined families. 

 We shall understand the characteristics by which 

 families are distinguished from each other when we 

 have studied more plants in detail, but for the present 

 it may be said that for this purpose we judge by 

 characteristics which are not likely to have been 

 much modified by external conditions, namely, the 

 position of the leaves (whether opposite or alternate), 

 the presence or absence of stipules, and, chiefly, the 

 general nature of the flower. 



FIG. 26 



PLUMERIA 



ACUTIFOLIA, 



Poiret 



