114 MOKI'HOI.OGICAL DEVELOPMENT. 



nny law ? And are these laws, if they exist, allied with one 

 another and with that to which the shape of the whole 

 plant conforms ? 



Descending to the components of these components, which 

 in developed plants we distinguish as leaves, there meet us 

 kinlred questions respecting their relative sizes, their rela- 

 tive shapes, and their shapes as compared with those of 

 foliar organs in general. Of their morphological differentia- 

 tions, also, it has to be asked whether they exemplify any 

 truth that is exemplified by the entire plant and by its larger 

 parts. 



Then, a step lower, we come down to those morphological 

 units of which leaves and fronds consist ; and concerning 

 these arise parallel inquiries touching their divergencies 

 from one another and from cells in general. 



Tne problems thus put together in several groups can- 

 not of course be rigorously separated. Evolution pre-sup- 

 poses transitions which make all such classings more or less 

 conventional; and adherence to them must be subordinata 

 to the needs of the occasion. 



214. In studying the causes of the morphological 

 differentiations thus grouped and prospectively generalized, 

 we shall have to bear in mind several orders of forces which 

 it will be well briefly to specify. 



Growth tends inevitably to initiate changes in the 

 s^iape of any aggregate, by changing both the amounts of 

 the incident forces and the forces which the parts exert on 

 one another. With the mechanical actions this is obvious : 

 mitter that is sensibly plastic cannot be increased in mass 

 without undergoing a change in its proportions, consequent 

 on the diminished ratio of its cohesive force to the force of 

 gravitation. With the physiological actions it is equally 

 obvious : increase of size, other things equal, alters the 

 relations of the parts to the material and dynamical factors 

 of nutrition ; and by so affecting differently the nutrition 



