98 MORPHOLOGY 



These special cells are in short and more or less branching fila- 

 ments that arise from the floor of the chamber and contain the chlo- 

 roplasts, and are thus freely exposed to the internal atmosphere. This 

 remarkable apparatus is one of the specialized features of the Mar- 

 chantiaceae. In the center of the epidermal dome, roofing each air 

 chamber, there is developed a chimney-like air pore. In the ventral 

 region the tissue is composed of colorless cells, and the ventral epidermis 



Fig. 227. — Marchantia: section through young air chamber (a), showing the special 

 chloroplast-containing cells (c), the epidermal roof (c), and the air pore (/>); the cells 

 beneath the air chamber (5) also contain chloroplasts ; the rhizoids and scales on the 

 ventral surface are not shown. 



develops two kinds of rhizoids and two longitudinal rows of scales. 

 Upon the dorsal surface cuplike structures (cupules) are produced, 

 which contain special reproductive bodies (gemmae), which can develop 

 new gametophytes (figs. 228, 229). The gemma of Marchantia is a 

 discoid body with two notches on opposite sides, the growing points 

 being located at the bottom of the notches (figs, n 18, n 19). 



In the development of the gametophytc body there are usually three distinct 

 stages : (1) a filament of varying length ; (2) the development in the terminal cell of 

 an apical cell with two cutting faces, 1 by means of which the thallus begins to 

 broaden; and (3) the development of an apical cell with three or four cutting faces, 

 by means of which the thallus begins to thicken. 



Sex organs. — The sex organs are not scattered over the dorsal sur- 

 face, but are restricted to definite areas, these areas becoming disks of 



J The current names for apical cells are somewhat confusing. For example, an 

 apical cell with two cutting faces is called "two-sided"; one with three cutting faces 

 a "three-sided apical cell," etc. It is evident that in each case the free surface of the 

 cell forms another side, and that a "two-sided apical cell" is really three-sided; a "three- 

 sided apical cell" is really four-sided, etc. That there may be no confusion, we have 

 used the somewhat clumsy expression "an apical cell with two cutting faces," etc. 



