?.:J4 



BOTANY 



Order 3. Dietyotaeeae 



In this order there are only a few forms (e.g. Dictyota dickotoma, Fig. 8, p. 13). 

 The asexual spores, of which only two or four are formed in a sporangium, are non- 

 motile. The sexual organs are differentiated into oogonia and antheridia. Each 

 oogonium contains a single egg-cell, which it eventually ejects, and the antheridia 

 produce numerous spermatia or non-motile male cells without cilia. The process 

 of fertilisation has not as yst been observed. In the form of their spores and 

 spermatia the Dietyotaeeae resemble the fihodophyceae, from which, however, they 

 are distinguished by the absence of a trichogyne and by their characteristic fruit- 

 formation. 



Economic Uses. — The dried stalks of the officinal Laminaria digitata, forma 

 Cloustoni (Pharm. germ.), are used as dilating agents in surgery. Iodine is ob- 

 tained from the ash (varec, kelp) of various Laminariaeeae and Fueaeeae, and 

 formerly soda. Many Laminarias are rich in mannite (e.g. Laminaria saccharina), 

 and are used in its production, and also as an article of food by the Chinese and 

 Japanese. Species of Alaria are used as an article of food in the Polar regions. 

 The larger Phaeophyceae are utilised also as manure. 



Class VIII 



Rhodophyceae (Red Algae) 



The Rhodophyceae or Florideae, of which about 280 genera are 

 known, constitute, like the Phaeophyceae, an independent group of 

 Thallophytes, for whose phylogenetic derivation from the lower Algae 

 there is, as yet, no positive evidence. They are attached to some support, 

 and almost exclusively marine, and specially characterise the lowest algal 

 region on the coasts of all oceans, especially in temperate and tropical 



latitudes. A few genera (e.g. 

 HJ }A Batrachospermum, Lemanea, Hilde- 



m, A ,/. \<o 1 brandtia) grow in fresh-water 



streams. 



The thallus of the red Algae 

 exhibits a great variety of forms. 

 As in the brown Algae, there 

 are no single-celled forms like 

 those characteristic of the Sipho- 

 neae. The simplest forms are re- 

 presented by branched filaments 

 consisting of single rows of cells 

 (e.g. Callithamnion). In other cases 

 the branched filamentous thallus 

 appears multicellular in cross- 

 sections. In many other forms the 

 (e.g. Chondrus crispus, Fig. .259 ; 



Fia. 259. — Chondrus crispus. s, Oval cystocarps. 

 (£ nat. size. Officinal.) 



thallus is flattened and ribbon-like 



