TIIALLOPHYTES 55 



no means so bulky as many of the brown algae, but they are much more 

 diversified in form. In the simplest forms the body is a simple or 

 branching filament, which is monosiphonous or polysiphonous (see 

 p. 45), or it may be flat and filmy, or ribbon-like. The more complex 

 forms show an extreme differentiation of the body into branching stems, 

 leaves, and holdfasts. The species of Corallina resemble branching 

 coral on account of the abundant deposit of calcium carbonate in their 

 cell walls. The general hue of the plants is red or violet, sometimes 

 dark purple or reddish brown, the color being due to the presence of a 

 red pigment (phycoerythrin) that may be separated from the green 

 (which may or may not be chlorophyll). In addition to the presence of 

 phycoerythrin, one of the chief peculiarities of the group is the absence 

 of any ciliated cells, either swimming spores or gametes, which is a sur- 

 prising feature in so extensive an aquatic group. Another conspicuous 

 peculiarity is the method of sexual reproduction, which often results in 

 a very complicated life history. 



The Rhodophyceae seem to form an independent group, with no 

 evident connections, and are certainly the most specialized of algae. 

 The Florideae so nearly include all of the red algae that the name is 

 often used as synonymous with Rhodophyceae. It is impossible to 

 present adequately the maze of forms, and three of the best known are 

 selected as illustrations, two representing the simpler forms and simpler 

 sexual reproduction, the third representing the more complex forms 

 and complex sexual reproduction. 



Before presenting these types, mention may be made of a small group that com- 

 bines certain characters of brown and red algae. 



Dictyotales. — This is a small group of marine forms, of uncertain affinities, but 

 interesting on account of the combination of characters. They are sometimes 

 included among the brown algae, and sometimes among the red algae, so that they 

 may well be mentioned between these two groups. The body is a narrow, repeat- 

 edly dichotomous thallus, whose pigment contains a brown constituent suggesting 

 that characteristic of the brown algae. Sporangia are developed on the surface of 

 the thallus, and each sporangium produces four non-motile spores (letraspores), , 

 conspicuous character of the red algae. The antheridia occur in clusters on the 

 surface of the thallus, and produce sperms with a single terminal spiral cilium, a 

 character that belongs neither to brown algae nor to red algae. The oogonia also 

 are clusters of superficial cells, each one discharging a single egg, a character sug- 

 gesting the egg-discharging habit of Fucus. In such a case, the comparative value 

 of characters must be estimated, and perhaps tetraspore formation is to be regarded 

 as the most important among those given. If this be true, the Dictyotales are to be 

 considered as an aberrant group of red algae. 



