THE VARIOUS GROUPS OF PLANTS 25 



Genera (sing, genus) : the members of each genus usually look 

 alike in a number of features and constitute one or more 



Species : these represent the smallest unit of classification in 

 general use, being those whose members show a broad similarity. 

 Biologists differ in their conception of what constitutes a species, and 

 indeed the term is scarcely capable of satisfactory definition. How- 

 ever, for the great majority of animals and many plants a species is, 

 roughly speaking, constituted by all those individuals which are able 

 to interbreed among themselves but are unable to breed, at least at 

 all freely, with members of other groups. The individuals of a 

 species are by no means identical but form a more or less variable 

 population in which some entities are often recognizable as subspecies 

 (written subsp. or ssp.), or as still more subordinate varieties (written 

 var.) or formae (written f.). 



Even as species are divisible into subspecies, so are the major 

 groups often divided into subphyla, subclasses, etc. The individual 

 is the ultimate unit, but inasmuch as no two individual plants can 

 be exactly identical, any more than two individual persons can be, 

 the smallest recognizable unit of classification is the hiotype, con- 

 sisting of all those individuals which have the same genetical make-up. 

 Thus most species consist of a large number of biotypes that differ 

 slightly in their inheritance. 



The scientific name of each species is normally made up of two 

 Latin or latinized words of which the first is the name of the genus 

 to which it belongs and the second is its own specific epithet, usually 

 having some descriptive or historical connotation. The initial letter 

 of the first, or generic, name is always capitalized, that of the specific 

 epithet nowadays being customarily left ' small '. Unfortunately, 

 English or other ' popular ' names are too unreliable to employ at 

 all widely, particularly because the same name is apt to be used for 

 different plants in different places or by different people, and also 

 because it is undesirable to have the same plant known under different 

 names in different places or sometimes even in the same place. 

 Moreover, it is confusing to have more than one combination of 

 names in use for a single entity, and so the Latin one is agreed upon 

 and employed internationally by scientists. 



We will now consider briefly what seem for our purpose to be 

 the main classes of the plant kingdom, each being treated under the 

 primary heading of the phylum (division) to which it belongs. The 

 sequence followed is probably indicative of evolutionary history in 

 broad outline. After a brief general account of the characters of 



