THE ALGAE 



177 



the segments (Fig. 168) and a series of smaller distal cells which cut off 

 laterally cells to form the new cortex. No cortex is formed on the joint 



cells. 



Growth may therefore be said to be apical, and the cells at the apices 

 of the terminal segments are thin- walled and delicate. It is only further 

 back that the encrustation of lime forms by deposition on the cortical cells. 

 As there is no cortex at the joints they remain uncalcified. This deposit is 

 due to the abstraction of Carbon dioxide from solution in the sea water during 

 photosynthesis, which leads to the dissociation of the Calcium bicarbonate 

 in solution and the deposition of insoluble Calcium carbonate on the surface 

 of the plant. Non-calcified Algae apparently escape encrustation by the 

 continuous sloughing of mucilage from the surface. 



As a general rule there are three growing points on each terminal 

 segment ; the central one, which continues the growth of the shoot, 

 producing fresh segments, and one formed on each side, which may 

 develop into reproductive organs or may give rise to two lateral branches. 



Anther- 

 idia 



Sexual Reproduction 



Whether the conceptacle is destined to contain antheridia, carpo 

 gonia or even tetrasporangia, its develop- 

 ment is similar, though as we have seen 

 the final shape is somewhat different. 

 Development begins at the apex of a 

 branch. A group of cells at the apex 

 of the growing segment are richly supplied 

 with cytoplasm. These cells are termed the 

 disc cells, and those lying at the periphery 

 continue to divide and grow up around the 

 reproductive organs, which develop at the 

 centre of the disc, leaving only a small 

 ostiole at the apex and enclosing these 

 organs in a well-formed conceptacle. 



The Antheridium 



In the formation of the antheridia each 

 disc cell divides into two unequal parts by 

 a wall which cuts off a small terminal cell, 

 which is the antheridial mother cell. 

 From this, antheridia arise in twos or 

 threes, forming a close layer over the base of 

 the conceptacle (Fig. 169). These antheridial 



cells now elongate, and their cytoplasm and nuclei migrate to their upper 

 ends leaving the basal part as a long thin tail. These structures become 

 detached from the mother cells and apparently function as spermatia, but 

 their exact nature is uncertain (Fig. 170). 



7 



Fig. 169. — Corallina officinalis. 

 Longitudinal section of an 

 antheridial conceptacle. 



