332 POtrsiDATIONS OP BOTAITY 



to the strongly drying character of the air in which they 

 grow (due partly to its rarefaction), and to the low temper- 

 ature' which they must endure every night. 



401. Aquatic Vegetation. '■ — Plants which live wholly in 

 water often need a less complicated system of organs than 

 land-plants. True roots may be dispensed with altogether, 

 as in many seaweeds, in most fresh-water algae, and in 

 some seed-plants.' A few such plants have mere hold- 

 fasts that keep them from diifting with the waves or the 

 current. Sometimes roots may, as in the duckweeds 

 (Fig. 220), serve the purpose of a keel and keep the 

 flat, expanded part of the plant from turning bottom up. 

 The tissues that give strength to the stems and leaves of 

 land-plants are not usually much developed in submerged 

 aquatics, since the water supports the weight of such 

 plants. In some algse, as the common rockweed or blad- 

 der-wrack (Fig. 188), the weight. of the plant is admi- 

 rably biaoyed up by.largp air-bladders. Transpiration is 

 done away with, and whatever carbonic acid gas or oxygen 

 is absorbed or given off passes directly through the cell- 

 walls into the interiors - of the cells. Generally water- 

 plants do not reach lany great size, but some species are 

 the longest of known plants, Maerocystis, the great kelp 

 of the Pacific Ocean, attaining, it is said, the length of a 

 thousand feet or more. In spite of the moderate size of 

 most algae the total bulk in the various oceans must be 

 extremely large. The Sargasso Sea alone, in the Atlantic 

 Ocean, reaches most of the way from the Bahamas to the 

 Azores and extends over seventeen degrees of latitude. 

 The whole area is occupied by a nearly compact mass of 

 floating seaweed. 



