ACKOGENS. 



51 



consisting of two genera, Chora and Nitella. In 

 the latter the stem is a simple tube, but in Chara the 

 central or axial cell is surrounded in a spiral manner 

 by others. In Chara, also, the stem is incrusted by 

 carbonate of lime. Fig. 20 will sufficiently illustrate 

 their forms. The points on the axis, or stem, from 

 which the branchlets spring, are called nodes, and the 

 intervening parts are the internodes. 



These stone-worts illustrate the manner of growth 

 in the type of Acrogens. Each internode is formed 

 by the growth and elongation of single cells. The 

 terminal bud is also formed by a single cell, which 

 subdivides into two. One of the latter forms the in- 

 ternode, while the other subdivides into lateral cells, 

 which by continual division produce the branchlets. 

 After a time the terminal cell in the latter is incapa- 

 ble of further division, but in the stem the process 

 continues indefinitely. (Fig. 21.) 



These stone-worts are 

 also reproduced by cer- 1 

 tain organs which grow 

 at certain parts of the 

 axis. These organs are 

 of two kinds, oval spor- 

 angia, or spore-fruits, 

 and antheridia, or or- 

 gans which contain fila- 

 ments corresponding to ^ Fig. 21. 



