49° 



BOTANICAL GAZETTE 



[DECEMBER 



specialized elements. These cells differ in the formation of their 

 cell plate, the wall lies somewhat diagonally, and the cell differs 

 from the other epidermal cells in shape, size, and content. This 

 type he finds in the Schizaeaceae, Equisetum, Azolla, Lycopodium, 

 Phylloglossum, Isoetes, Selaginella, and in Nymphaeaceae. Snow 

 (21) finds that no definite length can be given as the limit for the 

 formation for hair development; that in some roots the average 

 length of piliferous cells is less than that of the smooth cells, but 

 that the differential elongation of the epidermal and cortical cells 

 is important, and that hair formation depends upon their ratio, 

 that is, between the capacity of the epidermal cells to elongate 

 and their ability to do so. 



Bardell (i) comes to the same conclusion as Snow. A few 

 of the measurements given by Bardell are shown in table I. 



Plant 



Pisum sativum 

 Zea mais 



Length cortical 

 cell (mm.) 



40 

 124 



Haired epider- Hairless epider- 

 mal ceU (mm.) mal cell (mm.) 



161 (-71) 

 139 (-4°) 



66 (- 3) 



69 = 



55 (+15) 

 o 



5S 



191 (—40) 

 172 (- 7) 

 77 (+ 8) 

 77 (+ 8) 

 55 (+15) 

 33 (3) 

 62 



It is difficult to draw the conclusion which is given, for in the 

 one case a difference of 40 mm. in length between epidermal and 

 cortical cells calls forth a root hair; while in another the same differ- 

 ence accounts for the absence of a root hair, and measurements 

 equal to, or greater than, or less than, seem to account for haired 

 or unhaired epidermal cells. 



Succession.— Schwarz (20) found that hairs were always 

 produced in acropetal succession, while Devaux (2) asserted that 

 new hairs could appear among the old ones; whereas Haberlandt 

 (8) agrees with Schwarz (20), emphasizing the fact that no new 

 hairs ever arise among existing ones. 



Position of the nucleus.— Haberlandt (8) noted the posi- 

 tion of the nucleus in Brassica alba to be at the tip. In Pisum 



