GROUP I. THALLOPHYTA : ALG^E. 



239 



uudifferentiated forms the body is termed a ccenobium. Even 

 the most highly organised forms attain but a low degree of his- 

 tological differentiation, amounting (as e.g. 'in the Fucacese) 

 only to a distinction between peripheral assimilatory tissue and 

 central conducting tissue : in some of the Laminariacese the con- 

 ductiug-tissue has the form of sieve-tubes. 



Morphology. The body may be entirely undifferentiated ; this 

 condition is most common in the unicellular forms, but it also 

 occurs among the multicellular (e.g. Volvox) ; or it may present 

 a distinction of base and apex (e.g. Rivularia) ; or it may be 

 differentiated into root and thalloid shoot (e.g. Botrydium, Fucus) ; 

 or into root, stem, and leaf (e.g. Cladostephus, Chara, Polysiphonia). 



The undifferentiated body (thallus), as also the thalloid shoot, 



A 



xjy. 

 e. 



FIG. 133. Growing-points of Algae. A Apical growing-point, with apical cell, of 

 StypoeauloTi rcoparium ( x 30). B Intercalary growing-point (where the transverse lines are 

 close together) of Desmarestia ligulata in longitudinal section (x 60). C Apical growing- 

 point, with apical cll, of Cltaitopteris plumosa (x 40: afterFaulkenberg). 



presents great variety of form: it may be spherical, or filamentous, 

 or a flattened expansion, and its symmetry may be multilateral, 

 isobilateral, or dorsiventral. 



The growth in length of the thallus or of the shoot is effected in a 

 variety of ways. It may be either apical or intercalary (Fig. 133.) 

 In cellular plants the apical growth is effected either by a single 

 apical cell (e.g. Characese, Sphacelariese, Fucacese, Dictyota, Fig. 

 106, most Rhodophycese) ; or by a marginal series of apical cells 

 (e.g. Coleochseteae, some flattened Rhodophyceae) ; whereas in those 

 coenocytic plants (Siphonoidese) w r hich grow apically, there is no 

 apical cell, but an apical mass of embryonic protoplasm. In some 

 .cases of intercalary growth there is no growing-point, all the cells 



