APPENDIX A 



THE PLANT KINGDOM 



In classifying plants according to degrees of relationship, certain features 

 are more useful than others. The more important of these are briefly 

 discussed below. It should be pointed out, however, that they are of 

 unequal value, and that their applicability and usefulness vary from 

 group to group. 



1. Cell Structure. One of the simplest groups of plants (the blue- 

 green algae) is made up of cells which with few exceptions have no nuclei. 

 There is considerable reason to believe that they are rather closely related 

 to the bacteria, which also lack definite nuclei. Other instances of likeness 

 in cell structure are similarly interpreted as indicative of relationship. 



2. Arrangement of Cells. In some of the very simple plants every 

 cell division is followed by separation of the daughter cells, so that each 

 plant consists of a single cell. In others the cells remain together in pairs; 

 or larger numbers of cells form accumulations that may be arranged into 

 filaments, sheets of cells, or three-dimensional cell groups of various 

 shapes. Although all these types of cell arrangement are found in several 

 rather distantly related groups of thallophytes, within each of these 

 groups they often indicate relationship of species and genera. 



3. Presence or Absence of Particular Organs or Tissues. Whole plant 

 groups may be characterized by the presence or absence of some particu- 

 lar vegetative organ or tissue. Thus the presence of true roots in all 

 modern pteridophytes is an indication that these plants are more closely 

 related among themselves than are any of them to the mosses or other 

 rootless plants; it likewise indicates a closer degree of relationship to the 

 spermatophytes (which also have roots) than to the groups of plants in 

 which roots are lacking. Again, the absence of tracheal tubes (vessels) 

 from the wood of most gymnosperms indicates that these plants are more 

 closely related among themselves than they are to the angiosperms, in 

 which tracheal tubes occur. 



4. Similarity of Reproductive Structures. Throughout the plant king- 

 dom the reproductive mechanism, including the structure of the reproduc- 

 tive organs, affords one of the best and most used means of determining 

 relationships. The vegetative structures are much more likely to show 



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