106 



PLANT STRUCTURES 



mass, it is a dome-shaped mass, the inner cells of the hemi- 

 sphere having become sterile. This central group of sterile 

 cells which is surrounded by the ar- 

 chesporium is called the columella, 

 which means "a small column." 



In a moss called Sphagnum there 

 is the same dome-shaped archespori- 

 um with the columella, as in An- 

 thoceros, but it is relatively smaller 

 on account of the more abundant 

 sterile tissue (Fig. 88). 



In the highest Mosses the arche- 

 sporium becomes very small as com- 

 pared with the sterile tissue (Fig. 

 89). A foot, a long seta, and an 

 elaborate capsule are organized from 

 the sterile tissue, while the arche- 

 sporium js shaped like the walls of 

 a barrel, as though the dome-shaped 

 archesporium of Sphagnum or An- 

 thoceros had become sterile at the 

 apex. In this way the columella is 

 continued through the capsule, and 

 is not capped by the archesporium. 



This series indicates that after 

 the sporogonium begins as a simple 

 spore case (Riccia), its tendency is 

 to increase sterile tissue and to re- 

 strict sporogenous tissue, using the 

 sterile tissue in the formation of the 

 organs of the sporogonium body, as 

 foot, seta, capsule walls, etc. 



Among the Green Algse there is 

 a form known as Coleochwte, whose 

 body resembles those of the sim- 

 plest Liverworts (Fig. 90). When 



Pig. 89. Young sporogoni- 

 um of a true moss, show- 

 ing foot, seta, and young 

 capsule, in which the ar- 

 chesporium (darker por- 

 tion) is barrel-shaped, and 

 through it the columella is 

 continuous with the lid. — 

 After Campbell. 



