DISGUISES OF LEAVES, CONCLUDED 109 



words, an organ is a part of an organized (living) body directly asso- 

 ciated with the vital functions, as are the heart and lungs of ani- 

 mals. A member is a distinct or integral external part, as a wing 

 or a stamen, especially one which is not directly concerned in the 

 maintenance of the vital functions. In plants, many parts are at 

 once both organs and members, as the leaves; when one is consider- 

 ing these parts from the standpoint of morphology or form, it is 

 well to speak of them as members, but in a book upon physioiogy 

 they mighf be spoken of as organs. 



1186. We are now prepared to understand that the old idea of 

 the "complete" leaf — one which comprises blade, petiole and stip- 

 ules — is simply an inference. It is not a type or unit in nature, 

 of which the "incomplete" leaves are modifications. The "com- 

 plete" leaf is itself the exception, perhaps, and is only one of the 

 many types of variation which occur in the leaf -member. 



118c. The idea of leaf, as defined by an arbitrary position as- 

 signed to it upon the stem, comprises a most heterogeneous set 

 of members, as we have seen. When we say that spines are modi- 

 fied leaves merely because they occupy the position of leaves, we are 

 going beyond our knowledge. It is only now and then {as in the 

 case of the barberry) that we have the evidence of intergradient 

 forms. For purposes of definition, it is well to say that such and 

 such spines occupy positions which leaves normally occupy ; but be 

 yond this it is not often safe for the beginner to go. 



118d!. Advanced study of morphology and embryology may show 

 the true nature of any member, and it is generally able to dis- 

 tinguish clearly between leaves and stems ; and may be the only 

 conclusive resource in eases which are very difficult of interpreta- 

 tion. 



Suggestions. — The pupil should now challenge every leaf, spine, 

 tendril and scale he meets, for each one means something. He should 

 tD.ake observations to determine how many of the plants which he 

 commonly sees have stipules, petioles and blades, and how many 

 lack one or more of these parts. 



