CELLS AND TISSUES 17 



in Lactuca, the guard cells and epidermal cells are equal in 

 height. 



In the development of a stoma, the dermatogen cell which is to 

 produce it usually divides into two unequal daughter cells, one of 

 which becomes the initial cell of the stoma and the other an epider- 

 mal cell. Then the initial cell, which may be designated as the 

 stomatal mother cell, is bisected by an anticlinal wall, the two 

 daughter cells becoming the guard cells. This usually occurs in 

 young epidermal cells shortly after they have been cut off by the 

 dermatogen; but the formation of stomata in a given region does 

 not occur simultaneously, and a progressive series of develop- 

 mental stages may frequently lie in close proximity to one another. 



De Bary (7) has described three distinct types of stomatal 

 development, which are here briefly summarized. In type (i), 

 as in Allium, the initial cell is the stomatal mother cell which divides 

 to form the two guard cells, their opposing walls separating schi- 

 zogenously to form the stomatal opening. In type (x), as in the 

 first, the initial cell is the mother cell of the stoma; but, shortly 

 after it is differentiated, accessory cells are cut off from the adjacent 

 epidermal cells and run nearly parallel to the guard cells. This type 

 occurs in most Gramineae, including Zea and Triticum. (Fig. 5.) 

 In type (3), the initial cell is not the mother cell of the stoma, but 

 undergoes one to several successive divisions which result in the 

 formation of the mother cell and one or more accessory cells. The 

 manner in which the initial cell is divided and the new cell walls 

 are laid down is variable; but a common sequence is found in 

 Raphanus, where the new anticlinal walls are formed successively 

 in three directions and cut off a series of accessory cells which 

 surround the mother cell. Other examples of this type occur in 

 Apium, Ipomoea, and many of the Solanaceae. (Fig. 6.) 



Epidermal Hairs. — The epidermal cells may form directly or 

 give rise by division to various types of absorptive, protective, 

 glandular, or non-glandular hairs. These may be conical and 

 pointed, globular and capitate, unicellular or multicellular, and 

 unbranched or variously branched. 



Root Hairs. — In root tips, the epidermal cells immediately 

 above the zone of elongation develop as root hairs by the extension 

 of the outer wall to form an elongated tube. The hair first appears 

 as a rounded protuberance involving the whole or a part of the 

 outer wall; and, by rapid elongation, develops into a unicellular 



