100 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



and sponge-like habit so different from that of most AJgte. 

 But species have their appointed tracts, even as genera. 

 The common Fucus vesiculosus, for instance, which is so 

 characteristic of our coasts at mid-tide, is not known in the 

 Mediterranean except as drift, though it grows and even fruc- 

 tifies there in floating masses, assuming, sometimes, peculiar 

 forms. The distribution of species is, however, greatly modi- 

 fied by the nature of the currents. Sub-tropical species, 

 therefore, which occur only at the most southern extremities 

 of England, occur on the side of Ireland exposed to the 

 warm currents of the Atlantic as high as 53° ; and to the 

 same cause are due, most probably, the broad forms of Desma- 

 restia in Loch Swilly, identical with those on the Spanish 

 coast, though search might probably be made in vain at 

 intermediate points. Seasons, too, have more influence than 

 might be expected on the growth of Algae. Dr. Harvey 

 has observed, for instance, that Padina Pavonia is much 

 influenced by comparative warmth, though it has a wide 

 geographical range ; and such species as grow in shallow 

 pools are more likely to be affected than deep sea species, 

 for such isolated spots, in certain states of the tide, will, 

 sometimes, attain a marked increase of temperature. 

 Slight changes of temperature have, in fact, a greater in- 

 fluence on Algae than on most other plants. In general, 

 they are impatient of extremes or of exposure to strong 

 light, as may be easily verified by any frequenter of our 

 coasts. Some species are so delicate that a very short 

 exposure is sufficient. Shallow pools, which are capable of 

 being heated on the ebb of the tide, rarely afford Algaa in good 

 condition. 



81. It might be expected that most of the seaweeds which are 

 common on the western coast of Europe, would be more or 

 less so on the opposite coast of America, and this is true 

 to some extent ; but though Fucus vesiculosus and nodosus 

 are characteristic forms of the American coast, Fucus serratus 

 is entirely wanting, and other species, almost equally common 

 with that in England, are either unknown or extremely rare. 

 These species are, however, compensated by several kinds of 



