IPOMOEA BATATUS 



511 



The Mature Leaf. — At maturity, the mesophyll is much less 

 compact and there are numerous large intercellular spaces in the 

 spongy region and between the palisade cells. (Fig. 169.) The 



—sto in 



Fig. 2.69. Transection of base of foliage leaf through midvein : a c, accessory cell ; col, 

 collenchyma; g c, guard cell; g s, glandular scale; / ph, inner phloem; mx, metaxylem; 

 /i/j, outer phloem ; /)^/, palisade; /»a-, protoxylem. 



secretory cells are not easily detected, resembling large air cavities; 

 but, unlike intercellular spaces, they are not directly connected 

 with adjacent air chambers owing to the retention of the original 

 cell wall. 



The epidermal cells are irregular in size and have a definite 

 cuticle. Stomata are more numerous in the lower than in the upper 

 epidermis and are surrounded by 



the guard cells and two or more . i/^ v XM .r-3 c 



accessory cells. In the develop- 

 ment of the stoma, the epi- 

 dermal initial is divided by the 

 formation of successive walls 

 which are oriented in such a 

 way that several accessory cells 

 are formed which surround the 

 stomatal mother cell. (Fig. 

 2.-J0, A.^ The mother cell pro- 

 duces the two guard cells by a sto /«/stomatal Initid 

 final longitudinal division, and 



the stoma is formed by a separation of their opposed primary 

 walls along the middle lamella. The number of accessory cells 

 which may be formed is commonly three or four, and two of these 

 abut the guard cells. (Fig. xyo, B.) 



a c 



Fig. 170. A, early stage in stomatal forma- 

 tion ; B, mature stoma with guard cells and 

 accessory cells. Numerals i, i ; 1,2.; 3,}; 

 indicate sequence of wall formation : a c, ac- 

 cessory cells ; g c, guard cells ; sto, stoma ; 



