PART IV. CLASSIFICATION. 



times consists 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, Myrionema, Pylaiella) ; the body is frequently more or less 

 clearly differentiated into root and shoot, and in some cases (e.g. Clado- 

 stephus, Cheetopteris) the shoot is differentiated into stem and leaf ; adven- 

 titious roots are very generally developed. 



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

 forms (e.g. Ectocarpus, etc.) it is filamentous and branched, the filament con- 

 sisting of a single row of cells (monosiphonous) ; in others it is filamentous, 

 consisting of several coherent longitudinal rows of cells (poly$iplionous}\ 

 in the most highly developed forms 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. Laminariacese). 



Growth in length may 

 be effected without a de- 

 finite growing-point, all 

 the cells being merismatic 

 (e.g. generally in Ectocar- 

 pacese) ; or there may be 

 a definite growing-point, 

 which may be apical, with 

 an apical cell (e.g. Sphace- 

 lariese) ; or the growing- 

 point may be intercalary, 

 either sub - apical (e.g. 

 Chordaria), or more or 

 less basal (e.g. Laminari- 

 acese). The division of 

 the apical cell, or of the 

 initial cells, of the grow- 

 ing-point takes place only 



Fia. 153. - Longitudinal section through three inter- 

 nodes of a sexual plant of Cladostephus verticillatus 

 (Ectocarpaceae) : a gametophyll ; the larger appendages 

 are foliage-leaves. (x60: after Pringsheim.) 



in one plane, the transverse. The segments thus formed undergo division 

 either only transversely (monosiphonous forms), or longitudinally (poly- 

 siphonous), or in several planes. 



The sporangia are in all cases unicellular. In the simple filamentous 

 forms they are somewhat enlarged and rounded cells, either intercalary in 

 position (e.g. Pylaiella), or terminal, occupying the place of a lateral 

 branch, and generally sessile (e.g. Ectocarpus, etc.). In the more bulky 

 thalloid forms, the sporangia may be merely developments of single 

 superficial cells (e.g. Laminariacese) scattered singly or in groups (sori) 

 over the whole surface. In others again they are borne as lateral branches 

 on hair-like outgrowths from the superficial cells. In certain cases, where 

 the shoot presents differentiation into stem and leaf (e.g. Cladostephus), 

 the sporangia are borne on specialised leaves, sporophylls (Fig. 154). 



The gametangia are in all cases multicellular. each cell constituting a 



