
PLANT GEOGRAPHY 459 
Floating plants of cold temperate countries are all of small 
size, but they may be abundant enough to cover the surfaces of 
small ponds. In warmer regions floating plants reach larger size 
(Fig. 5), and the water hyacinth (Fig. 72) may obstruct naviga- 
tion in sluggish streams. Floating plants are provided with well- 
developed air spaces (Fig. 71), which make the plants buoyant 

Fig. 515. Lotus (Nelumbium nelumbo) projecting out of the water, and 
Pistia stratiotes floating on the water, in Laguna de Bay, Philippine Islands 
and at the same time serve as passageways in which oxygen can 
diffuse to the roots. The roots hang down in the water and serve 
as a counterpoise which helps to keep the plant right side up. 
Swamp vegetation. Around the edges of quiet bodies of fresh 
water, and in shallower water than that in which plants with 
floating leaves occur, there is often a conspicuous development ot 
swamp composed largely of erect monocotyledonous plants with 
roots under water or in saturated soil and with shoots extend- 
ing into the air (Fig. 515). Such plants contain conspicuous air 
