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. 



FIG. 1 60. 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 



