THE THALLUS OF THE HIGHER ALG.S. 2/ 



form maybe understood l)y comparing with one of the fila- 

 mentous alg;"e a member of an isolated order of green fresh- 

 water algce, the Characecs. 



Characeae. 



32. The order. — These plants constitute an ouflying group 

 of considerable antiquity, having no near relatives living, yet 

 showing in the vegetative body some structural resemblance 

 to the filamentous algae, while, as a whole, their external 

 form imitates quite closely that of the higher plants (figs. 

 35, 36). The species of Chara and Xitella (the two genera 

 which make up the bulk of this order) are found in almost 

 every temperate region, growing in dense masses submerged 

 and rooting in the mud in quiet waters. They reach a height 

 of 10-75 cm. 



33. External form. — The plants agree in having a central 

 axis, at certain points of which* arise lateral outgrowths of 

 two kinds. One kind forms a circle of branches, nearly like 

 the main axis, except that their growth is limited. These 

 themselves bear branches of simpler structure. The primary 

 whorled branches are the so-called " leaves," and the second- 

 ary ones which these bear are the so-called " leaflets." 



Just above one of the "leaves" in each whorl is pro- 

 duced a branch precisely like the main axis, which has, like 

 it, unlimited growth. 



34. The main axis. — In Nitella the axis consists of alter- 

 nately long and short cells, a very short cell occurring at each 

 point (" node ") where branching occurs. The long cell 

 extends from one " node " to another. This "internodal" 



* Commonly called nodes, and the intervals internodis. These terms, 

 imposed from analogies with the seed-plants, are entirely misleading from 

 a morphological point of view, as are also the names "leaves" and 

 " leaflets," applied to certain divisions of the axis, but they have become 

 so fixed that it is difficult to avoid their use. 



