222 PART III. THE CLASS fFICATION OF PLANTS. 



The colouring-matters phycocyanin, pbycophaein, and pbycoerythrin, can be 

 extracted by means of water ; they thus differ from chlorophyll, which is in- 

 soluble in water. The presence of chlorophyll in the Cyanophyceas, Phseophycese, 

 and Rhodophyceffi, can be proved by extracting the other colouring-matters with 

 water ; tbe plants then assume a green colour. 



Structure. The body may be unicellular ; or coenocytic and 

 unseptate (as in the Siphonaceae), or incompletely septate (Clado- 

 phoraceae) ; or multicellular. The unicellular forms either exist 

 singly, or a number may be held together in a colony by a mucila- 

 ginous common cell- wall, either as a filament (e.g. some Desmidieas) 

 or a mass (palrnelloid Protococcaceae, Syngeneticae, Chroococcaceae). 

 In some of the multicellular forms (e.g. Spirogyra, Pandorina, 

 Ulva) all the cells of the body are quite similar ; at first vegeta- 

 tive, they eventually become reproductive, so that there is.no 

 distinction between nutritive and reproductive cells : in these 

 histologically un differentiated forms the body is a ccenobium (see 

 p. 92). Even the most highly organised forms attain but a low 

 degree of histological differentiation, amounting (as e.g. in the 

 Fucaceae) only to a distinction between peripheral assimilatory 

 tissue and central conducting tissue : in some of the Laminariaceae 

 the conducting-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 

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

 or into root, stem, and leaf (e.g. Caulerpa, Cladostephus, Sar- 

 gassum, Chara, Polysiphonia). 



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

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

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

 isobilateral, or dorsiveritral. 



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. In 

 cellular plants the apical growth is effected either by a single 

 apical cell (e.g. Characeae, Sphacelariese, Fucaceae, Dictyota, Fig. 

 140, most Rhodophyceae) ; or by a marginal series of apical cells 

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

 coenocytic plants (Siphonoideaa) which 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 



