48 MANUAL OF BOTANY 



marked off from a colourless internal mass. The sexual organs 

 are antlieridia and oogonia, and are grouped together in sori. The 

 asexual ones are gonidangia, which occur in clusters or scattered 

 over the surface of the thallus. The gonidia and both forms of 

 gamete are nonciliated and motionless. In some forms the re- 

 productive cell, whether sexual or asexual, gives rise to a plant 

 like the parent ; in others a kind of filamentous body is de- 

 veloped, recalling the protonema of the Bryophyta, to be de- 

 scribed later. The adult plant arises as an outgi-owth from some 

 of the cells of this filament. 



The spores are often produced in groups of four in the 

 sporangia. In this point and in the unciliated character of 

 the antherozoids the Dictyotacese approach the group of the 

 Ehodophycea?. 



The Syngenetic.e form a group which is in marked con- 

 trast with the rest of the Phseophyceae. They are unicellular 

 organisms, and resemble the Cyano^jhyceae in forming colonies 

 in consequence of the cells not separating after division, but 

 remaining surrounded by their difHuent mucilaginous cell-walls. 

 They have no sexual reproduction, but bear asexual spores, 

 which are ciliated. 



Sub-Class IV. — EHODOPHYCEiE. 



This sub-class includes a large number of forms, nearly all of 

 which are marine. They are found growing mixed with olive- 

 green or brown forms at low- water mark and a little beyond, 

 and in deeper water they occm- almost alone. The fresh-w^ater 

 forms are few and belong only to two or three families. Some 

 marme forms are parasitic on other red Alga; ; others are 

 epiphytic. Some members of this family have the thallus 

 encrusted or impregnated with carbonate of lime, forming 

 structures which superficially resemble the animal corals. 



The morphological differentiation of the group is greatly in 

 excess of their anatomical complexity. They are all multi- 

 cellular, and generally form a much-branched thallus, which 

 may be flattened and spreading, or may be filamentous. The 

 segments of the shoot may in many cases be called leaves, and 

 a root can usually be fomid. 



The histological differentiation is much slighter than in the 

 last group. The filamentous forms sometimes show nothing more 

 than a single row of cells ; sometimes there are several such 

 rows, which may be covered by a kind of cortex composed of 

 small cells. In these cases the rows of the filaments do not 



