POLYSIPHONIA FASTIGIATA 199 



1. The section is circular, since the thallus is cylindrical. 



2. That it is limited externally by a clearly marked cuticuie, 

 and it will be remembered that as Ascophyllum, on which 

 it grows, is found about half-tide mark, the plant is exposed to 

 the air for several hours in each tide. 



3. The series of pericentral cells, of variable number. 



4. The single large central cell. 



5. Note especially the protoplasmic strands, which run from 

 the central cell to the several pericentral cells, traversing the 

 cell-wall. 



IV. Cut transverse sections of the frond of Ascophyllum^ so 

 as to pass through the insertion of one of the tuttSQiPolystphonta: 

 mount as before, and observe under a low power that elongated, 

 thick-walled cells produced from the base of the Polysiphonia 

 plant penetrate deeply into the thallus of the host, and thus 

 obtain a firm hold upon it, while their own strongly thickened 

 walls will explain further the strength of attachment. 



V. 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 dis- 

 turbed at certain points by dark spherical bodies, which lie 

 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." 



5. That they escape by rupture of the layer of pericentral 

 cells: note in older parts the vacant cavities whence tetraspores 

 have escaped. 



6. That the mature tetraspores are naked, spherical, and 

 motionless protoplasmic bodies. 



VI. Mount as before a small piece of a male plant taken in 

 April, and observe under a medium power 



