MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 129 



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 

 kindred 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 divergences from 

 one another and from cells in general. 



The problems thus put together in several groups cannot 

 of course be rigorously separated. Evolution presupposes 

 transitions which make all such classings more or less con- 

 ventional; and adherence to them must be subordinate to 

 the needs of the occasion. 



214. In studying the causes of the morphological differ- 

 entiations thus divided out 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 shape 

 of any aggregate, by altering both the amounts of the inci- 

 dent forces and the forces which the parts exert on one 

 another. With the mechanical actions this is obvious. 

 Matter 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 rela- 

 tions of the parts to the material and dynamical factors of 

 nutrition; and by so affecting differently the nutrition of 

 different parts, initiates further changes of proportions. 

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