46 A TEXTBOOK OF THEORETICAL BOTANY 



one another, the pear being Pyrus communis ; the apple, Pyrus mains ; and 

 the mountain ash, Pyrus auaiparia. Thus we see that the generic name 

 becomes comparable with the surname in human society, while the specific 

 name is equivalent to the Christian name. 



In order to assist in appreciating the rank of various terms it is customary 

 that all names of Families should end in -aceae and those of Orders in -ales, 

 while Sub-orders end in -ineae. The names of Phyla should end in -phyta. 

 Unfortunately this system has been adopted relatively recently and some 

 of the older names do not fit properly into the scheme, so that there are many 

 exceptions to these terminations. 



As knowledge increases there is a tendency to separate and differentiate 

 organisms in an ever-increasingly complex system. Critical work on any 

 group generally results in splitting up the species into a larger number and 

 as a result classification becomes more complicated. Moreover authorities 

 often differ as to the importance to be attached to particular features and 

 as a result their systems of classification vary. 



We are concerned here, however, with the main outlines of plant 

 classification, and about these substantial agreement exists. The following 

 scheme therefore is one which is generally accepted among botanists and it 

 offers a comparatively simple arrangement of the more important groups. 

 Much has been left out intentionally for the sake of simplicity, and only those 

 orders are mentioned of which a particular type will be subsequently con- 

 sidered in detail in this book. 



OUTLINE OF THE CLASSIFICATION OF PLANTS* 



I . Thallophyta. Plant body a thallus, with no distinction of stem, root 

 and leaf. Individuals mostly small with simple organization, many micro- 

 scopic. 



{a) Algae. Organisms possessing chlorophyll and sometimes additional 

 pigments. 



(i) Euglenophyceae. Motile protoplasts with no cellulose wall. 

 No sexual reproduction. 



(2) Chlorophyceae. Cellulose wall present. Chlorophyll only. 



Fresh- water and marine. 



(3) Cyanophyceae. Blue-green colour due to phycocyanin. 



Fresh- water and marine. 



(4) Phaeophyceae. Brown colour due to fucoxanthin. Ex- 



clusively marine. 



(5) BaciUariophyceae. Unicellular forms with silicified cell 



walls. Brown colour due to diatomin. Fresh-water and 

 marine. 



(6) Rhodophyceae. Red colour due to phycoerythrin. Pre- 



dominantly marine. 



. * ,^ full discussion of the methods and principles of plant classification will be included 

 in volume II. 



