488 PLANT GEOGRAPHY 



465. 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. 

 Many such plants have mere holdfasts that keep them from 

 being washed out of place. In the duckweeds (Fig. 355) the 

 roots answer the purpose of a keel and keep the flat expanded 

 part of the plant from turning bottom up. The tissues that 

 serve to strengthen the plant body are not much developed in 

 submerged aquatics, since the water supports most, if not all, 

 of the weight of the plant. Stomata are absent, and the absorp- 

 tion of carbon dioxide and giving off of oxygen go on directly 

 through the delicate cell walls, unprotected by an epidermis 

 (Fig. 354,^). Submerged aquatic seed plants occur in consider- 

 able abundance in sea water as well as in fresh waters, but the 

 marine forms do not include many species. 



466. Influence of rainfall in determining regions of vegeta- 

 tion. While the mean annual temperature and the extremes of 

 heat and cold, humidity of the air, force and direction of winds, 

 elevation above sea level, and nature of the soil are all factors 

 in determining the boundaries of regions of vegetation, there is 

 no factor more important than the annual rainfall. Of course 

 the rainfall itself is largely determined by several of the other 

 circumstances above mentioned. 



In the United States this varies greatly, the yearly averages 

 for some of the most important areas being about as follows : 



AVERAGE RAINFALL PER YEAR 

 REGION INCHES 



New England and Middle States 43 



Eastern Gulf States 55 



Ohio basin 44 



Missouri basin .31 



Rocky Mountains, middle of eastern slope .... 20 



Rocky Mountains plateau, middle 9 



Pacific slope, northern portion 37 



Pacific slope, southern portion 10 



