36 PLANT GEOGRAPHY 



The importance of water in plant-geography exceeds 

 in some respects that of heat, because its distribution is 

 more irregular. It exists in two main states, either as 

 invisible atmospheric moisture, or precipitated, whether 

 as dew, mist, rain, snow, running or standing waters, or 

 that in the soil. The amount of invisible moisture in the 

 air increases with the temperature; but the condition 

 of atmospheric moisture which is most important with 

 reference to plant-life is the saturation deficit, or the 

 amount of moisture by which the air at any time or 

 place falls short of saturation. This regulates the 

 amount of transpiration or loss of water by the plant. 



Moist air is correlated with the same structures in 

 plants as is feeble illumination. Many shade-plants 

 (dryads or sdophytes) and submerged aquatic plants, 

 which are very similarly conditioned, have long inter- 

 nodes, smaller, thinner, and more transparent leaves, 

 with little palisade-tissue, vascular tissue, or mechanical 

 tissue, but with large intercellular spaces. Sun-plants 

 (oreads or heliophytes), on the other hand, being adapted 

 to dry air and, therefore, to a desirably limited transpira- 

 tion, are more compact in growth, with thick leaves, 

 often vertical, and small intercellular spaces in them. 



It is true that the greater part of the water taken in 

 by plants is derived by them, not from the invisible 

 moisture of. the air, but from precipitated moisture, 

 especially rain, which has entered the soil. Thus it is 

 upon the geographical distribution of rainfall that 

 many of the main features of the earth's vegetation 

 depend. In the immediate neighbourhood of the Equator 

 we have the "zone of constant precipitation," where a 

 very high rainfall is almost equally precipitated through- 

 out the year; and there we have the equatorial ever- 

 green forest-zones of the Amazon and Congo basins, 

 known technically as the " hot rain forest " or Selva. 

 Over most of India, Further India, and the coasts of 

 Mexico, a heavy periodical, or " monsoon," rainfall, 

 confined to part of the year, produces the less dense 

 "monsoon forest"; whilst on the higher ground of 

 eastern Brazil, British East Africa, and northern 

 Australia we get a lower rainfall and the lower-growing 

 " hot thorn forest," or Caatinga, with many deciduous 

 trees. With a less heavy and less continuous rainfall in 

 tropical latitudes, as in the southern Soudan, Rhodesia, 



