42 PRACTICAL BOTANY. 



The whole is made up of cells of nearly equal size, arranged 

 in linear series, which correspond to one another in such a way 

 that the cells appear in fan-like zones. Here again there is no 

 single central series larger than the rest. 



E. Observations may also be made with advantage on speci- 

 mens of Lemanea, a genus of dark olive-green, fresh-water Algae, 

 included in the Floridese : these plants grow attached to rocks in 

 mountain streams. Note especially 



1. The simple filamentous, and branched proembryo, similar 

 in many respects to that of Chara : by means of this the plant is 

 attached to the rocks. 



2. The more complex " reproductive thallus " which is pro- 

 duced as a lateral branch upon it : in the character of the apex, 

 and in its mature structure, it is not unlike that of some species 

 of PolysipJionia : it has clearly marked nodes. 



Transverse and longitudinal sections should be cut, from which 

 it will be seen that the " reproductive thallus " consists of a 

 central series of elongated cells, which traverses a cavity filled 

 with mucilage, and is connected by lateral struts, at points be- 

 tween the nodes, with the cortical tissue : the latter consists of 

 relatively large internal cells, and relatively small superficial ones. 



VII. Having distinguished with a lens, or under a 

 low power, a specimen bearing tetraspores, mount a 

 portion of it as before directed (taking care that young 

 branches as well as mature ones are represented), and 

 examine under a medium or high power. Note 



1. That the regularity of the tissues of the thallus is 

 disturbed at certain points by dark spherical bodies 

 lying embedded in the tissue below the pericentral cells ; 

 these are the tetraspores. 



2. That they decrease in size as the apex of the 

 branchlet is approached. 



3. That they have no clearly definite arrangement. 



4. That each undergoes a division into four, hence the 

 term " tetraspore." 



