232 Siplwnales 



(3) Rock and coral-reef Caulerpas. These are more especially forms of C. racemosa, 

 although other species also occur. They are found in both exposed and sheltered 

 localities, forming compact patches consisting of numerous entangled rhizomes firmly 

 fixed to the rocks by richly branched rhizoids. In the more sheltered places the plants 

 are larger in all respects and the rhizomes more vigorous. When attached to coral reefs 

 Borgesen states that in his experience it is only to the dead parts of the corals. 



It would appear from Borgesen's researches that there are two groups of Caulerpas, 

 those with leaf-like bilateral shoots and those with radial shoots. The former are derived 

 from the latter, and the difference is apparently caused by different degrees of light and 

 exposure. The radial Caulerpas are essentially inhabitants of the littoral region in 

 shallow water, being adapted to live in intense light, whereas the bilateral forms become 

 more pronounced in deeper water which is often muddy and where the intensity of the 

 light is much reduced. At depths of 20 30 metres the leaf-like segments of the ' foliar 

 shoots ' are quite distichous and even the branches may all be situated in the same plane. 



In the absence of any form of reproductive organs the relationships of Caulerpa. are 

 not at all clear. It may possibly be related to Bryopsis since Correns finds that the 

 cell-wall consists of a similar substance in each case, and Wille has declared that the 

 characteristic trabeculae of Caulerpa sometimes occur in Bryopsis. 



Quite a number of fossil impressions have been attributed to the Caulerpacese, some 

 being described under the name Caulerpites and others as species of Caulerpa. Caulerpites 

 cactoides Goppert was described from Silurian and Cambrian strata, and Catderpa 

 Carruthersi Murray from the Kimmeridge Clay. These and many similar impressions are 

 exceedingly doubtful, and it is more than probable that some are casts of animal origin 

 rather than of plant origin (vide Seward, '98). 



Family Codiacese. 



The thallus in this family is of very varied form and in some genera is 

 encrusted with calcium carbonate (Halinteda, Penicillus, etc.). The plant 

 consists of a much-branched co3nocytic tube, the thread-like branches of 

 which are so interwoven that they give rise to a thallus with a definite 

 external form. The interweaving of the branches is sometimes rather loose 

 (AvrainviUea, Boodleopsis, Tydemania, Penicillus) and at other times so 

 compact as almost to form a tissue which has received the name of ' plecten- 

 chyma' (Udotea, Halimeda, Cladocephalus, etc.). In most cases the thallus 

 is differentiated into a medullary region of interlacing branches and a cortical 

 region in which short branches stand out at right-angles to the long axis. 

 This cortical region often forms a superficial limiting layer which has been 

 termed the ' palisade layer.' 



The branches of the coenocyte have only a limited apical growth and, 

 after a certain length has been attained, one or two lateral outgrowths arise 

 behind the now dormant apex of the branch, each outgrowth forming a new 

 branch of about the same length as the parent-axis. The branching is 

 usually dichotomous, either in one plane or in alternate planes, although 

 trichotomous branching occurs in the basal part of Chlurodesmis comosa, in 

 Boodleopsis, in Tydemania, in Penicillus Sibogw, in the capitulum of 

 P.dumetosus,&nd in the flabellum of Udotea conglutinata and U. glaucescens; 



