RELATION TO ENVIRONMENT 



497 



with great tenacity, and these plants are very slow to dry, in strong 

 contrast to the rapid loss of water from most submersed plants when 

 exposed to the air. 



Surf Algae. Algae which grow upon rocks are often exposed to 

 the heavy beating of the surf, and such species are provided with 

 very efficient holdfasts, which are only torn away with great force. 

 The tissues of these surf-plants are also very tough, so that they are of 

 a leathery or cartilaginous consistence, and are perfectly adapted to 

 withstand the buffeting of the waves without injury. Sometimes, 

 where these grow only partly submersed, as in the Sea-palm (Postel- 

 sia) of the Pacific coast, the tissues are rigid enough so that the plant 

 maintains an upright position in the air, which is rarely the case 

 with Algge (PL VIII). 



In the larger seaweeds, also, a much better development of the 

 plant-body occurs than among the simple fresh-water forms. As we 

 have seen, many of these large 

 Seaweeds develop leaves, or special 

 organs for carbon-assimilation, and 

 these may be brought to the sur- 

 face by means of plants, so as to 

 offer the most favorable exposure 

 to the light. These large Algae 

 have, in short, adjusted themselves 

 more perfectly to the peculiar con- 

 ditions existing in the ocean, than 

 have any other plants, and they 

 are preeminently the characteristic 

 types of marine shore-vegetation. 



Plankton Forms. Very different 

 are the conditions prevailing in the 

 open sea, where there are myriads 

 of plants forming part of the 

 floating life, or " Plankton " of the 

 ocean. This floating vegetation, 

 being the source of food for most 

 of the inhabitants of the ocean, is of 

 course of the greatest importance. 

 While a small number of the 

 higher Seaweeds, such as the Gulf- 

 weed and some related forms, may F IG- 465. Pelagic Diatoms. A, Chaeto- 

 be found floating and vegetating 

 far from land, this is exceptional, 

 and the bulk of the floating plant life is made up of Diatoms, 

 Peridineee, and other minute, often microscopic, forms. Many of 

 these show very beautiful adaptations to this floating life, having the 



2K 



ceras boreale. B, C, Planktoniella 

 Sol. (After SCHUTT.) 



