April, 1909.] The Classification of Plants, V. 489 



THE CLASSIFICATION OF PLANTS, V. 



John H. Schaffner. 



In a previous paper the entire plant kingdom was classified 

 into seven fundamental divisions or subkingdoms, representing 

 the great successive stages in the evolution of plants as a whole. 

 These groups do not show genetic relationships but simply steps 

 in the upward evolutionary motion. But there is a principle of 

 segregation operative in the organic kingdom as well as one of 

 progression. The whole plant kingdom thus comes to be a series 

 of greater and smaller divergent lines or branches. In a group 

 of nonsexual organisms every line of descent is a single line which 

 diverges from another line at a definite point, but in a sexual 

 group, where interbreeding goes on freely, there is an interaction 

 throughout the whole mass and the scheme of descent resembles 

 an elongated net with greater and smaller meshes. The whole 

 progressive network of descent of a group may, however, also be 

 represented by a line. When individuals or groups of individ- 

 uals arise which become sterile to other members of the general 

 group a new line is segregated, so that for the larger groups the 

 diagram of descent must be quite similar whether of sexual or 

 nonsexual forms, even though the diagram for individuals is 

 fundamentalh^ different in the two cases. 



As a convenient guide to memory, the scheme of relationships 

 may be represented graphically by a tree with greater and 

 smaller branches. Every branch thus recognized, whether large 

 or small, is characterized by some peculiarity which remains 

 dominant in all of the individuals and groups of the branch. Or 

 in other words, as Bessey* has said: "Every phylum is the 

 result of a development which differs from that which preceded 

 it because of the incoming of a new dominant idea." The ntimber 

 of great branches or phyla recognized depends somewhat on the 

 views of the particular systematist making the classification. It 

 is not always easy to distinguish fundamental genetic characters 

 from those which are merely progressive, and may be developed 

 in various unrelated groups. Among the characters which do 

 not represent genetic relationships, when considered by them- 

 selves, but which have been developed independently in various 

 lines ma}^ be mentioned the following: Origin of sexuality, dif- 

 ferentiation of gametes, passage from a unicellular to a filamen- 

 tous condition, differentiation of the filament with base and 

 apex, loss of chlorophyll with development of parasitism and 

 saprophvtism, development of unisexual gametophytes, loss of 

 sexuality, origin of heterospory, development of complex leaf 



* Bessey, Chaei.ksE. The Phyletic Idea in Taxonomy. Science N. S. 20: 91-100, 1909. 



