GROUP I. THALLOPHYTA: ALGM : PH2BOPHTC&S. 



259 



sists of a flattened dorsiventral branched filament, the branches of which are 

 often coherent into a disc which adheres to the substratum by the ventral 

 surface and bears vertical shoots on its dorsal surface (e.g. Ectocarpus, Myrio- 

 nema, Pylaiella) ; the body is frequently more or less clearly differentiated into 

 root and shoot, and in some cases (e.g. Cladostephus, Chastopteris) the shoot is 

 differentiated into stern and leaf ; adventitious roots are very generally de- 

 veloped. 



The body presents a considerable variety of structure. In the simplest forms 

 (e.g. Ectocarpus, Pylaiella, Choristocarpus, etc.) it is filamentous and branched, 

 the filament consisting of a single row of cells (monosiphonous) ; in others it is 

 filamentous, consisting of several coherent longitudinal rows of cells (poly- 

 siphonous ; e.g. Giraudia, species of Sphacelaria) ; or it is monosiphonous and 

 partly or completely invested by a secondary parenchymatous tissue forming a 

 sort of cortex (e.g. Desmarestieae, Spermatochnus) : or it is polysiphonous 

 with a cortex (e.g. Sphacelaria scoparia, Chaetopteris, Cladostephus) ; or, 

 finally, it consists of parenchymatous tissue frequently differentiated into a small- 

 celled cortex and a medulla of large cells elongated parallel to the long axis of 

 the plant (e.g. most Encoeliaceae, Laminariaceae) ; the body frequently becomes 

 hollow, a central cavity being formed (e.g. Asperococcus, Scytosiphon, Colpo- 

 menia, Striaria, Chorda). A body of monosiphonous or polysiphonous structure 

 is articulated longitudinally into segments corresponding to the cells which 

 constitute the filament or filaments ; this is also generally the case when such 

 a body is corticated (e.g. Chsetopteris, Arthrocladia). 



Growth in length may be 

 effected without a definite 

 growing-point, all the cells 

 being merismatic (e.g. gener- 

 ally in Ectocarpaceaa and 

 Eucceliaceaa), in which case 

 growth is, as a rule, limited; 

 or there may be a definite 

 growing-point, which may 

 be apical, with an apical cell 

 (e.g. Sphacelarieae, Dictyo- 

 siphon, Spermatochnus) ; or 

 the growing-point may be 

 intercalary, either sub-ap'cal 

 (e.g. Chordaria) as also in 

 Desmarestiaceaa \(Fig. 160), 

 Cutleriacese, and Sporoch- 

 naceaa where the growth 

 is trichothallic (p. 223), or 

 more or less basal, (e.g. 

 Laminariaceae). The divi- 

 sion of the apical cell, or 



of the initial cells, of the growing-point takes place only in one plane, the trans- 

 verse. The Fegments thus formed undergo division either only transversely 

 (monosiphonous forms), or longitudinally (polysiphonous), or in several planes. 



FIG. 18E. Longitudinal section through three inter- 

 nodes of a sexual plant of Cladostephus verticillatus : 

 a gatnetophyll ; the larger appendages are foliage- 

 leaves. (x50: after Pringsheim.) 



