GROUP I. THALLOPHYTA : ALG2E : PHJ30PHYCEJ;. 



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, Cheetopteris) the shoot is 

 differentiated into stem 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 pareochymatous tissue forming a 

 sort of cortex (e.g. Desmarestiese, 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 Eneffiliaceaa, 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 ease 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 EctocarpaceaB and 

 Encceliaceas), 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. Sphacelariese, Dictyo- 

 siphon, Spermatochnus) ; or 

 the growing-point may be 

 intercalary, either sub-apical 

 (e.g. Chordaria) as also in 

 Desmarestiaceas [(Fig. 160), 

 Cutleriaceae, and Sporoch- 

 nacese where the growth 

 is trichothallic (p. 223), or 

 more or less basal, (e.g. 

 Laminariaceaa). 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 segments thus formed undergo division either only transversely 

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



FIG. 185. Longitudinal section through three inter- 

 nodes of a sexual plant of Cladostephus verticillatus: 

 a gametophyll; the larger appendages are foliage- 

 leaves. (x50: after Pringsheim.) 



