PRINCIPAL GROUPS OF PLANTS. 



8i 



DORSiVENTRAL. Froiii the lower colorless surface unicellular 

 rhizoids arise (Fig. 47, h). The upper surface consists of several 

 layers of cells containing chlorophyll which give the green color 

 to the plant. 



Vegetative propagation may ensue by the lower portion of 

 a branch dying and the upper portion continuing as an inde- 

 pendent plant. Or special shoots, known as gemm.^i, may arise 



Fig. 46. Dichotomously branching thallus of the common liverwort {Marchantia 

 polymorpha) showing near some of the margins the cup-like depressions in which gemmae 

 are borne (c), and several archegoniophores (a). 



either on the margin of the thallus or in peculiar cupules, which, 

 when detached by rain or other means, are capable of growing 

 and producing a new plant. 



In addition the thallus body produces both antheridia and arch- 

 egonia (Fig. 46) which may arise on special stalks above the sur- 

 face. After fertilization of the egg-cell, which completes the work 

 of the sexual generation of gametophyte, the sporophyte develops, 

 6 



