THE ALGAE 163 



The cells of the pericentral siphons form regular tiers, each of which is 

 equal in length to the corresponding cell of the central siphon. The thallus 

 thus has the appearance of being divided into nodes and internodes, which is 

 characteristic and distinctive. 



The pericentral cells of different tiers are at first unconnected, but con- 

 nections soon develop by a peculiar method. A small triangular cell is cut 

 off at a lower corner of a pericentral cells but remains connected to its parent 

 cell by a protoplasmic thread. This small cell fuses with the pericentral cell 

 immediately below it, thus establishing a protoplasmic connection between 

 the two tiers (Fig. 152, c to f). 



In older branches of the thallus the pericentral cells divide again repeatedly 

 by periclinal walls, cutting off a number of external layers of cells which 

 divide transversely into smaller cells, forming a cortex. These cells are all 

 connected together by protoplasmic threads and are comparable to a system 

 of contracted branches, the outermost cells of which form a superficial 

 assimilating layer. 



The thallus is attached at the lower end by means of a system of short 

 filaments or rhizoids, whose distal ends are swollen into attaching discs 

 fixed to the rock. 



Polysiphonia grows quite commonly around the British coasts below 

 high tide mark, both on rocks and in pools. In colour it varies from clear 

 red to purple. One common species, Polysiphonia fastigiata, lives as a 

 semi-parasite on the Brown Seaweed, Ascophyllum nodosum. We have here 

 an example of an hXgz which may be in process of becoming a parasite, for 

 Polysiphonia fastigiata can still live an independent life, though it has been 

 shown that plants living apart from the host are smaller and less healthy 

 than those which are attached to Ascophyllum. 



Sexual Reproduction 



In Polysiphonia male, female and asexual reproductive organs are all 

 borne on separate plants which are morphologically identical. 



The Antheridium 



The antheridia arise on short branches which are formed in clusters near 

 the tips of the main filaments (Fig. 153). Each antheridial branch consists of 

 a central trichoblast filament, which cuts off laterally a number of pericentral 

 cells, each of which further cuts off a number of antheridial mother cells, 

 from each of which are formed three or four antheridial cells. From the 

 latter spermatia are liberated. The central filament is quite covered and 

 hidden by the mass of colourless spermatia (Fig. 154). 



The Carpogonium 



The procarp consists of a row of three or four cells arising from a peri- 

 central cell. The last cell of the branch enlarges and a thin terminal portion 



