382 FOUNDATIONS OF BOTANY 
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 alge, and in 
some seed-plants. A few such plants have mere hold- 
fasts that keep them from drifting 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 alge, as the common rockweed or blad- 
der-wrack (Fig. 183), the weight of the plant is admi- 
rably buoyed up by large 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 any great size, but some species are 
the longest of known plants, Macrocystis, 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 alg 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. 
