36 STRUCTUKAL BOTANY 



We see, then, that although Cryptogams and Phanero- 

 gams appear to differ so completely from one another, 

 we can yet successfully compare them together and 

 determine the relations between their organs. 



Organs which resemble each other in their development 

 and their place in the life-history, so that we regard them 

 as morphologically the same organ, are said to be 

 homologous one with another. Organs, on the other 

 hand, which are morphologically different, but are 

 adapted to the same physiological function, are said to 

 be analogous. To go back to our old illustration in the 

 introduction to Part I. (p. 4), the tuber of a potato is 

 homologous with a branch of the stem but analogous with 

 a fleshy root such as that of a carrot. In our comparison 

 between Selaginella and a Gymnosperm we have aimed 

 at establishing the homologies of the various organs ; 

 such comparisons are essential in order to determine the 

 relationships of different groups of plants, for it is only 

 by tracing the homology or morphological equivalence of 

 organs that we can form any idea of the possible modifica- 

 tions which may have taken place during the course of 

 descent. 



The proof that the reproductive organs in Flowering 

 and in Flowerless Plants are homologous was due to a 

 German botanist named Hofmeister, and is one of the 

 greatest discoveries ever made in morphology. We 

 have every reason to believe that the Flowering Plants 

 are descended from Cryptogams, which resembled 

 Selaginella in having two kinds of spores. Their actual 

 ancestors, however, which, no doubt, have been extinct 

 for millions of years, may very probably have been in 

 other respects quite different from Selaginella. 



One point remains : we have not yet considered the 



