106 



MORPHOLOGY 



tions or perform similar labor, without respect to origins, 

 are analogous. Thus a leaf-tendril is analogous to a branch- 

 tendril, but the two are not homologous. 



224. There are five tests by means of which 

 we may hope to determine what a given part 

 is: (1) by the appearance or looks of the part 

 (the least reliable test) ; (2) by the 

 position of the part with relation 

 to other parts — its place on the 

 plant; (3) by comparison with 

 similar parts on other plants 

 (comparative morphology); (4) by 

 study of intermediate or connecting 

 parts; (5) by study of the develop- 

 ment of the part in the bud or as it originates by means of 

 the microscope (embryology). The last test can be applied 

 only by the trained investigator, but it often gives the most 

 conclusive evidence. Even with the application of all these 

 tests, it is sometimes impossible to arrive at a definite con- 



159. Leaf and 

 cladophyllaof 

 asparagus. 



160. Leaves of 

 asparagus. 



161. Fern-like leaf-branches of a 

 greenhouse asparagus. 



elusion as to the origin or morphology of a part. For ex- 

 ample, it is not yet agreed whether most cactus spines 

 represent leaves or branches, or are mere outgrowths of the 

 epidermis (as hairs are). 



