Nema- 

 thecium 



154 A TEXTBOOK OF THEORETICAL BOTANY 



the development of a procarp branch and an auxiliary cell is present before 

 fertilization. A fairly extensive carposporophyte tissue may be produced. 



Most of the species are free-living, but the group includes two genera, 

 HarveyeUa and Choreocolax, which are parasitic on other Rhodophyceae. 



We shall consider in detail one example of the group, Chondrns crispus. 



Chondrus crispus (Irish Moss or Carragheen Moss) 



This is the only species of the genus which is found in British waters. It 

 shows considerable variation in the form of the thallus, but is always divisible 

 into a shoot and an attaching organ. It is very plentiful around our coasts, 

 occurring mostly just above low tide mark wherever the shore is rocky. 



The attaching organ is 

 a flat, not very thick plate of 

 tissue, which adheres very 

 closely to the substratum and 

 anchors the seaweed firmly 

 to its support. In colour it 

 is faintly red, and it may to 

 some extent assist in the as- 

 similation of food, though its 

 primary function, apart from 

 anchorage, is the storage of 

 elaborated food material. It 

 grows by extension of the 

 margin and covers anything 

 which may be attached to the 

 rocks, extending into holes 

 and crevices, thus increasing 

 its hold on the rock surfaces. 

 From this attaching organ 

 numerous upright shoots are 

 produced (Fig. 142). These 

 are at first undivided and more or less round in section, but after growing 

 to a height of about 3 cm. they begin to expand and to dichotomize into 

 two equal branches, each of which forms a flat frond of varying width and 

 thickness. By successive dichotomies a fan-shaped thallus is produced which 

 may lose its basic regularity of division. The degree of division and the 

 breadth of the segments have been employed as criteria for separation into 

 a number of varieties. These are, however, of little value since they are by 

 no means constant. In general, it may be said that the higher up the beach 

 the plant grows the smaller and narrower the thallus becomes. Similarly the 

 colour of the fronds varies from dark red to pink, and even to greenish-brown 

 if one compares plants near low tide with those nearer high tide mark. Pre- 

 sumably therefore the darker red colour is associated with deeper water and 

 less light, a feature which has already been mentioned in connection with 

 Batrachospermum. 



Fk; 



142. — Chomirus crispus. Form of entire 

 plant. Half natural size. 



