Tunicata (p. 665). Although most plants 

 display no locomotion, quite a few of the 

 lower forms are motile. The symmetry of 

 most animals is bilateral (p. 624). But many 

 like typical plants, are radially symmetrical. 

 Generally plants can keep on growing, 

 •whereas animals stop growing when a certain 

 size is achieved. But again this is not always 

 true. In short, the only basis for placing an 

 organism in a certain category is to consider 

 all available evidence bearing on its origin, 

 morphology, and physiology in an effort to 

 establish its true and natural relationships. 

 At one time quite a number of animals, espe- 

 cially certain Coelenterata (p. 630), which 

 have a plantlike form, were placed in the 

 Plant Kingdom. In fact, the true position of 

 the Coelenterata was determined only later, 

 when they had been studied much more 

 carefully. 



Taxonomic Categories. As is shown in 

 Appendix I, modern taxonomy subdivides 

 each kingdom into major and minor cate- 

 gories. These, in descending order of mag- 

 nitude, are called the Phylum, the Class, 

 the Order, the Family, and finally the Genus 

 and Species. However, various additional 

 subdivisions may also be employed. 



Each individual kind of organism is iden- 

 tified by a double name. This binomial sys- 

 tem, which was introduced mainly by Lin- 

 naeus, specifies the genus and species of this 

 organism. The name, Homo sapiens, for ex- 

 ample, indicates that we humans belong to 

 a small recently evolved group of manlike 

 creatures, the genus Homo. Also it shows 

 that we represent only one kind, the species 

 sapiens, which is slightly but definitely dif- 

 ferent from Homo neanderthalensis, or 

 Homo soloensis — manlike creatures that be- 

 came extinct just before the dawn of the re- 

 cent epoch. Or the name Acer saccharum, 

 which designates the sugar maple, shows that 

 this plant has maplelike leaves, flowers, fruits, 

 etc., which identify it as a member of a small 

 group, the genus Acer. But also the name 

 shows that this plant is of a particular kind 

 (the species saccharum), from which maple 



The Plant Kingdom - 593 



sugar is derived. Ordinarily, interbreeding 

 can occur only within the limits of one spe- 

 cies; but sometimes it also can occur between 

 two different species — if these are related very 

 closely. 



In any brief account, the larger categories 

 of the taxonomic system cannot be defined 

 very accurately. Suffice it to say that each 

 phylum represents a major subdivision in 

 the plant or animal kingdom. Presumably 

 the members of a phylum originated from 

 the same ancestral stock, very early in the 

 course of evolution. Each phylum, in turn, 

 is subdivided into classes; the classes into 

 orders; the orders into families; and the 

 families into genera; each genus being com- 

 posed usually of several species. Also there 

 are other categories, such as subkingdom, 

 subphylum, subclass, etc., especially in groups 

 containing a great diversity of forms. 



Some Important Questions. Although the 

 main purpose of this survey of the Plant 

 Kingdom is to provide for a recognition of 

 the major groups of plants, a number of 

 other important questions will keep cropping 

 up. What is the structural organization of 

 the group? Is it unicellular, colonial, or truly 

 multicellular? How far has the differentia- 

 tion of special tissues progressed? How about 

 the vascular tissues that are so essential to 

 the life of land species? How did the higher 

 plants manage to leave the water, which was 

 the age-old environment of the primitive 

 species ever since the beginnings of life? 

 What is the pattern of the reproductive 

 structures and processes, and how does this 

 pattern help one to determine the evolu- 

 tionary relationships of a certain group or 

 to understand how reproduction became 

 adapted to land conditions? What fossil evi- 

 dence is available, and how does this bear 

 upon questions of antiquity and relation- 

 ship? What is the importance of the group 

 in nature and in human affairs? Does it con- 

 tribute significantly to the supplies of food, 

 construction materials, clothing, drugs, etc.? 

 Is it important in relation to soil fertility, 

 water pollution, or disease? These and many 



