THE THALLUS OF THE HIGHER ALG&. 3 1 



branches from the node above grow downward and others 

 from the node below grow upward until they meet and inter- 

 lock about the middle of the internode (fig. 38). Thus, the 

 cortical cells are not produced by division from the large 

 central cell which they cover and stiffen, but simply grow over 

 it and become united with it at a very early age, increasing 

 with its growth and undergoing division at the same time, so 

 that each cortical branch becomes multicellular. 



36. Apical cell. — The axis and all its branches, in both 

 genera, are produced by the growth of a single apical cell of 

 hemispherical form {x, fig. 38). The segments, successively 

 cut off by partition-walls from its base, each divide a second 

 time. One of the cells so produced increases rapidly in size, 

 and becomes the internodal cell, while the other, by succes- 

 sive divisions and differentiation, forms the node and its ap- 

 pendages. In those branches which show unlimited growth 

 the apical cell retains its hemispherical form until death j but 

 in the divisions with limited growth ("leaves") it becomes 

 pointed and ceases to cut off segments from the base. 



37. Rhizoids. — The structures by which the Characeae are 

 held in place are adapted to penetrate the soft mud of the 

 ponds and lakes in which they grow. From the nodes near 

 the base of the axis arise numerous colorless rhizoids, often 

 of considerable strength through thickening of the cell-walls. 



The thallus shows decided increase in specialization of 

 members. This is accomplished, however, with a minimum 

 of differentiation in the cells of which the body is composed. 



Polysiphonia. 



In the marine algae a still higher specialization of members 

 is reached. One of the red seaweeds may be used to show 

 the gradual advance in complexity. 



38. External form. — The body of Polysiphonia, a branch- 

 ing alga (fig. 39) which grows in abundance upon rocky 



