i9i 2] THOMPSON— STOM ATA OF CRETACEOUS CONIFERS 65 



tion, it is difficult to imagine how these structures could effectually 

 serve as guard cells. 



The proper conception of the arrangement and homologies of 

 these parts may be most easily obtained from an examination of 

 living forms. The conditions existing in Agathis bomensis are 

 represented in fig. 6, which is a photograph of a vertical section of 

 the base of the leaf of that species. The two conspicuous oval 

 cells almost in contact are the sunken guard cells. Inclined above 

 them, with their small extremities at the strongly cutinized surface, 

 are the accessory cells. Each of the latter is seen to have a slight 

 projection into the cavity some distance above the guard cells. 

 Viewed from the surface (fig. 7), the accessory cells are seen to be 

 jour in number surrounding the opening. From the same view- 

 Point, the guard cells (fig. 8) are seen to be two in number, and 

 arranged in the usual manner. 



of Frenclopsis are really accessory cell: 

 below them for the true guard cells. As 



guard 

 must 



that 



materi 



are not likely to have been preserved. 



man y 



able specimens. Fig. 3, which is a photograph of another stoma 

 m section, shows two well-preserved guard cells at the bottom of 

 the cavity into which the conical structures project. Fig. 4 shows 

 another stoma with unmistakable guard cells below the accessory 

 ce lls. In this figure the end of a projection from another acces- 

 sory cell has been cut off and appears in the center of the stomatic 

 cavity. In the majority of the stomata examined in section, no 

 guard cells can be distinguished; in others, fragments have been 

 preserved, especially the outermost wall, which appears to have 

 been more strongly lignified; in still others, the whole structure is 

 preserved in exactly the relations which one would expect in living 



material 



more difficult to observe the guard 



Vl ew, owing to the fact that they are covered by the extremely thick 

 accessory cells. This circumstance also entirely precludes their 

 reproduction by photograph. Nevertheless, examination of the 



