38 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



may here be comparable in several ways with that of most animals. 

 Many types of Brown Algae have an alternation of sexual and asexual 

 generations — usually of strikingly different sizes and forms, as in 

 the case of Pteridophytes {see pp. 55 et seq.). 



Although a few simple types occur in fresh water, the vast majority 

 of Brown Algae live in salt seas or in brackish lagoons and estuaries. 

 They are abundant in the tropics, but tend to be still more prominent 

 in colder waters, even persisting to the northernmost arctic shores. 

 Being often of considerable size, they are commonly the most 

 conspicuous features of many northern rocky shores between tide- 

 marks and for some distance below, forming extensive and often 

 almost pure ' beds '. These may cover and obscure the rocks or 

 boulders, to which the plants are attached by holdfasts so tough that 

 they are normally detached only during severe storms. Thereafter 

 they may be cast up in large piles upon the beach, or float until 

 they die and ultimately disintegrate. A few special kinds of Brown 

 Algae, however, seem able to live indefinitely in a detached floating 

 state, forming extensive masses ; of these masses the largest and 

 most notable characterizes the Sargasso Sea in the western Atlantic 

 Ocean. 



The distribution of the Brown Algae seems to be world-wide 

 wherever suitable sea-shores are found. Nor is their economic 

 significance negligible. Thus some are extensively harvested for 

 human food or animal fodder, particularly in eastern Asia, while 

 their use as manure after being cast up during storms is widespread. 

 The ash obtained by burning certain Kelps and Wracks, particularly, 

 is still in some places an important source of iodine and potassium. 

 Finally, owing to their peculiar food-storing and other biochemical 

 activities, Brown Algae are nowadays an important source of often 

 unique organic chemicals. An example of these is ' algin ', the 

 production of which runs into about a thousand tons annually in 

 the United States alone. 



Rhodophyceae : These are the Red Algae, or Red Seaweeds, 

 which tend to be more prolific in different species but are generally 

 less bulky and abundant as individuals than the Brown Algae. This 

 is especially the case in the seas of temperate and boreal regions, 

 where Red Algae may be little in evidence. They are characteristic- 

 ally red or purplish owing to the presence in the chloroplasts of 

 special pigments besides chlorophyll, although some may be greenish, 

 bluish, olive, or brown. The thallus is again very various in form 



