IRRITABILITY. 



387 



to light; B is a section of the corresponding internode of a "laid" plant 

 imperfectly exposed to light. 



A. 



FIG. 42 (after Koch), a, epidermis; b, cortical parenchyma; c, sclerenchyma ; 

 d, vascular tissue. 



It has been found, further, by Kraus, Koch, and Rauwen- 

 hoff,that the epidermal and parenchymatous cells of excessively 

 elongated etiolated internodes are much longer than those of 

 the corresponding normal internodes, and that they are also 

 rather more numerous. 



The following table, taken from RauwenhofFs work, illustrates the 

 relative development of the tissues and the length of the cells in normal 

 and etiolated internodes. The plant used was Polygonum cuspidatum. 



The numbers represent divisions of the eye-piece micrometer. 



252 



