CHAPTER IV 



CLASSIFICATION; THE SIMPLEST PLANT-FORMS 



IT is generally assumed that a real genetic relationship exists 

 among all plants, and the aim of a natural system of classification 

 is to express the degree of this relationship. An ideal classification 

 would represent the family tree of the vegetable kingdom, but un- 

 fortunately such a classification is not to be hoped for, owing to the 

 complete disappearance of many plant-forms, which has resulted in 

 the survival of many isolated types that are only distantly related 

 to other known forms, and to which it is impossible, at present, to 

 assign a certain position in the system of classification. Among 

 such isolated groups may be mentioned the Diatoms and Characeae. 



Factors in Classification. In determining the degree of relation- 

 ship between plants, probably the general structure, or morphology, 

 is of the first importance ; but as certain parts, especially the repro- 

 ductive structures, are less subject to change from external con- 

 ditions, these less variable structures are, of course, especially 

 important in classification. Where plants are obviously closely 

 related, as, for instance, two species of the same genus, it is differ- 

 ences, rather than resemblances, that are considered in assigning 

 them their places. Where relationships are less obvious, it often 

 becomes necessary to study all phases of the development of the 

 plant its " life-history " in order to determine its affinities with 

 other forms. No single point of structure can be safely used alone, 

 and, so far as possible, all the structures must be considered. 



Ontogeny and Phylogeny. It is assumed that the life-history, or 

 " Ontogeny," of the individual repeats, to some extent, the evolution 

 of the race, " Phylogeny," and a study of the developing organism, 

 is often of the greatest importance in making out its relationship to 

 other and especially lower forms. All Mosses and Ferns, for exam- 

 ple, produce minute motile reproductive cells (spermatozoids), which 

 closely resemble similar cells among the Algae, and indicate that 

 these land plants have sprung from aquatic ancestors resembling the 

 existing Green Algae. 



The geological record, so far as it goes, is of very great value in 

 tracing the evolution of the vegetable kingdom; but unfortunately 



66 



