20 FUNDAMENTALS OF CYTOLOGY 



cells are elongating, it can be seen that a wave of elongation joasses from 

 the base toward the apex of the root, this wave at a given moment affect- 

 ing alike whatever portions of neighboring cells occupy the same level, 

 whether these portions are basal, median, or apical. The wave is some- 

 thing not dependent upon boundaries of the unit cells. Here, as in the 

 cucurbit fruits and coenocytes, the unity of behavior and of organization 

 inheres primarily in the whole rather than in the elements composing it. 



Correlation. — In every normal mass of protoplasm, whatever its 

 growth pattern or degree of differentiation, the many diversified activities 

 are so coordinated that it behaves as a consistent whole, or individual, 

 from the beginning of development onward ; without such harmony there 

 obviously could be no organism. How this harmony is maintained has 

 never been fuU}^ explained. Recent work on higher plants and animals 

 has shown that diffusible substances play an important role in this con- 

 nection, and something of the sort may well occur within the limits of 

 single cells. In multicellular tissues the fine protoplasmic strands 

 (plasmodesms) connecting neighboring protoplasts in all probability 

 facilitate correlation (Fig. 56). In animals the nervous system functions 

 as a specialized correlating mechanism. It has also been shown that 

 gradients in the rate of metabolism and in electric potential along the 

 various axes of symmetry are correlated with differentiation with respect 

 to these axes. Reversal of the electrical polarity ma}^ result in a reversal 

 of morphological pattern. Moreover, there is some evidence that in and 

 about an organ or developing embryo there is a characteristic pattern of 

 potential — a "field" — that exercises some measure of control over what 

 occurs within it. " Differentiation, upon such a view, is to be looked upon 

 as a setting up of new fields, each resulting from changes in size or position 

 during ontogeny or phylogeny." All this suggests that a physicochemical 

 explanation of correlation msLy be hoped for and that a more satisfactory 

 conception of organic "wholeness" may some day be attained. 



Tissues and Organs. — Since the fundamental processes of develop- 

 ment — growth, differentiation, and correlation — may occur normally in 

 the several protoplasmic growth patterns, it is scarcely adequate to define 

 a tissue as a group of cells and their products. Many tissues are just 

 this, but a better definition would be one applicable also to other patterns, 

 including the plasmodial state. 



Although we commonl}^ think of an organ as a specialized multicel- 

 lular structure because of familiar examples, the more comprehensive 

 definition, "any part of an organism performing some definite function," 

 is more nearl}' adequate if one remembers that a protozoan as well as a 

 metazoan may have distinct regional differentiations associated with 

 special localized functions. On the basis of function, which gives organs 

 their significance, the contractile region within a protozoan cell is as much 



