28 



E), a number of other types, such as tracheids, vessels, wood fibers, 

 sclerenchyma fibers, and sieve tubes (Fig. 4), all of which are characterized 

 by the peculiar ways in which their walls become modified through sec- 

 ondary and tertiary thickenings, and by the form and arrangement as- 

 sumed by the pits. (See p. 191.) The protoplasts may finally disappear 

 completely from wood cells, leaving a tissue or framework composed of 

 lifeless cell walls. 



FIG. 4. Differentiated cells from vascular plants. 



A, wood fiber with thickened wall. B, C, portions of tracheids with spiral and annular 

 thickenings. D, pitted tracheid. E, portion of sieve tube with adjacent companion cells. 

 F, face view of sieve plate shown in section in E. 



All of this variety of form and structure is conditioned by varied 

 functional activity on the part of different protoplasts : in the process of 

 cell differentiation morphological and physiological changes stand in the 

 closest relationship. All functional differences are accompanied by 

 chemical or physical differences of some sort in the protoplasm, but it 

 is mainly in the non-protoplasmic inclusions and secretions (including 

 the wall) rather than in any conspicuous structural changes in the 

 protoplast itself that cell differentiation is rendered visible in the case 

 of plants. Apart from differences in shape, amount of vacuolar material, 

 accumulated food, and other products of differentiation (see p. 133), 

 protoplasts performing widely different functions may appear much alike. 



Structural differentiation in connection with division of labor is very 

 striking in animal cells, which are destitute of such walls as plant cells 

 possess. The muscle cell shows many fine longitudinal fibrillae which 

 have to do with the cell's power of contractility. In certain muscles 

 these fibrillse are so segmented that the whole cell, or muscle, fiber, has a 

 transversely striped appearance (Fig. 5, F). The nerve cell (Fig. 5, 



