THE VARIETY OF PLANT LIFE 85 



A phylum is generally made up of species with some common 

 conspicuous and unique characteristic, such as the phylum of the 

 fungi, all of which lack chlorophyll. 



The characteristics which are used as the criteria of plant 

 relationships, and by which plants are grouped into species, 

 genera, etc. are of two types. One type is associated with meta- 

 bolic activities and the vegetative structures of the plant; the 

 other is connected with differences in reproductive structures 

 and habits. 



Vegetative characteristics which differentiate plants include 

 (/) the presence or absence of chlorophyll; (2) the relative com- 

 plexity of the plant body, whether unicellular, multicellular with 

 little division of labor, or multicellular with complex organs; {3) 

 the anatomy and habits of stems; and (4) the anatomy and types 

 of leaves, with their varying habits (deciduous or evergreen, for 

 example). Many vegetative characters are untrustworthy guides, 

 since such organs as stems and leaves vary greatly among mem- 

 bers of the same species, frequently responding to slight environ- 

 mental changes. 



Reproductive differences, being less often the result of 

 environmental influences, are more constant. Such character- 

 istics involve (7) the type of reproduction present, whether sexual 

 or asexual; (2) the character of the sporophyte or gametophyte 

 generation; {3) the type of reproductive organ, whether sporan- 

 gium, cone or flower; and (4) the type of associated reproductive 

 structure, such as fruits or seeds. Differences among species of 

 flowering plants are usually found to be restricted to variations in 

 flower structure (number, position and character of stamens or 

 petals), kind of fruit produced, or seed characteristics. 



On the basis of such criteria as these, species are grouped into 

 genera, genera into families, families into orders, orders into 

 classes and classes into phyla. Since the orders and classes are of 

 interest chiefly to the botanist, and often are such highly artificial 

 units that there is considerable difference of opinion as to just 

 what they slaould include, the emphasis in this discussion will be 

 placed upon genera, occasional families, and phyla. There are 

 approximately a dozen phyla in the plant kingdom; eight of 

 these will be discussed in the present chapter. 



