ACROGENS. 



face of the frond is seen by a low magnifying power to 

 be divided into diamond-shaped spaces, containing an 

 opening in each. On making a thin section, as in Fig. 43, 



FIG. 43. A. Portion of frond of Marchantia polymorpha seen from above, a. a. Loz- 

 enge-shaped divisions, b. b. Stomata seen in the center of the lozenges, c. c. Greenish 

 bands separating the lozenges. B. Vertical section of the frond, showing a. a. the dense 

 layer of cellular tissue forming the floor of the cavity d. d. b. b. Cuticular layer, forming 

 its roof. c. c. Its walls, f.f. Loose cells in its interior, g. Stoma divided perpendicu- 

 larly, h. Rings of cells lorming its wall. z". Cells forming the obturator ring. 



each of these stomata will be seen to form a sort of 

 shaft, or chimney, of four or five rings, or courses, of 

 cells, the lowest ring regulating the aperture into the 

 leaf-grottoes below. 



The spores of Marchantia are attached to elaters, or 

 spirally-coiled elastic fibers, whose extension scatters the 

 spores. 



3. The EQUISETACE^:, or Horsetails, are found in 

 most parts of the world, save Australia and New Zea- 

 land. They generally grow in wet places, sending up 

 shoots from a creeping stem, or rhizome. The cuticle is 

 remarkable for the great quantity of silica contained in 

 it. The particles of this mineral, each having a double 

 axis of refraction, are arranged in rows parallel to the 

 axis, and are beautiful objects under the microscope, 



with polarized light. The abundance of silica has led 

 10* 



