LIFE HISTORY OF A LIVERWORT 
215 
ized plant a blue-green alga, of the genus Nostoc (Fig. 160). 
On the ventral surface of the thallus slits occur in the 
epidermis. These are not stomata, and the intercellular 
spaces into which they open are fitted with a mucilaginous 
substance produced by a transformation of the adjacent 
cell-walls. This mucilage furnishes ideal conditions of 
food and moisture for the alga, which flourishes there. 
V* 
FIG. 160. Photomicrograph of a cross-section of a liverwort (Antho- 
ceros fusiformis) . The dark, oval area is a colony of a species of Nostoc, 
an alga that lives symbiotically in the tissues of the liverwort. (Micro- 
scopic preparation by M. A. Howe.) 
Whether the presence of the alga is of any advantage to 
the liverwort is not known, but apparently it is of no 
disadvantage. 
197. Vegetative Multiplication. Liverworts present 
many interesting devices for vegetative multiplication by 
the giving off or separation of a portion of the vegetative 
tissue, and the establishment of this separated piece as an 
independent plant. No group of plants excels the 
liverworts in their power to regenerate new individuals 
from pieces of the plant body. If the thallus is cut into 
