Z] THE VARIOUS GROUPS OF PLANTS 35 



the cell transversely while the other runs longitudinally along 

 one side. Two flagella are inserted where the grooves cross each 

 other — an undulating one which lies in the transverse groove and 

 appears to be largely responsible for the rotation of the organism, 

 and a more normal looking one running down the posterior portion 

 of the longitudinal groove and effecting movement forward. 



Nutrition is mainly by photosynthesis, food being stored as either 

 starch or oil. Not only is a reddish eye-spot frequently present, 

 but some types have colourless bodies and are saprophytic, while a 

 few, at least, ingest solid food and so are animal-like in their feeding. 

 As these are among the organisms that are on the border-line between 

 animals and plants, they are liable to be claimed also by zoologists. 

 Reproduction is chiefly effected asexually by the division of an 

 individual into two dissimilar halves (the cells are often markedly 

 asymmetric at first), after which each half regenerates'the missing half. 



Peridinians are very widely distributed in both fresh and salt 

 waters and may be especially abundant in the ocean. Thus in 

 arctic seas they tend at some times of the year to outnumber even 

 the Diatoms and, temporarily, to form the main constituent of the 

 plankton. Consequently they are an important source of food for 

 marine animals — including, ultimately, Fishes, Seals, and even the 

 greatest Whales. 



Phaeophyceae : This large group, commonly called the Brown 

 Algae or Brown Seaweeds, are characterized by their brown or 

 olive-green colour which is due to the chloroplasts containing a 

 special brown pigment in addition to chlorophyll. They are practic- 

 ally all marine, being among the most abundant and familiar sea- 

 weeds of temperate and more austral (southern) as well as boreal 

 (northern) coasts. The thallus is multicellular and usually attached, 

 but shows a very wide range of different forms, some examples 

 of which are shown in Fig. 8. Though sometimes slender and 

 filamentous, the thallus is more often complex. Frequently it is 

 relatively massive, being differentiated into a disk- or root-like organ 

 of attachment to tidal rocks or sea-bed objects, and a stem-like part 

 of varying length and thickness bearing a ribbon- or leaf-like portion. 

 This last may be branched or unbranched and is usually elongated 

 and flexible, streaming easily with the current or, in shallow water, 

 often floating. Such buoyancy is commonly aided by the inclusion 

 of air bladders, which are usually conspicuous and large enough to 

 * pop ' when trodden upon — as in the familiar Bladder Wrack (Fucus 



