170 I'LAXT STUniKS 



Before considering some of tlie common societies, it is 

 necessary to note some of the conditions wliicli determine 

 plant societies. Tliose things in tlie environment of tlie 

 plant which influence the organization of a society are 

 known as ecological factors. 



VA. Water. — Water is certainly one of the most im- 

 portant conditions in the envii'onment of a plant, and has 

 great influence in determining the organization of societies. 

 If all plants are considered, it will l^o noted that the amount 

 of water to whicli tlicy are exposed is exceedingly variable. 

 At one extreme ari' those plants which are completely 

 submerged; at the other extreme are those plants of arid 

 regions which can obtain very little water ; and between 

 these extremes there is every gradation in the amount of 

 availal)le water. Among the most striking adaptations of 

 plants are those for living in the presence of a great amount 

 of water, and those for guarding against its lack. 



<_)ne of the flrst things to consider in connection with 

 any plant society is the amount of water supply. It is not 

 merely a (juestion of its total ainnial amount, but of its 

 distriljution througli the year. Is it suiiplied somewhat 



uniformly, or is there alternating fl 1 and drought ? The 



natui'c of the water supply is also important. Ai'e there 

 surface channels oi- subterranean channels, or does the 

 whol<> supply come in the form of rain and snow -which 

 fall upon the area ? 



Anotlier impoi'tant fact to consider in connection with 

 the water supply has to do with the structure of the soil. 

 There is what may be called a wa,ter level in soils, and it is 

 iini)orta,nt to note the depth of tbis level beneath the sur- 

 face. In some soils it is verv iicai' (he surface ; in others, 

 su(-li as samly soils, it may lie some <listani.'e lieneath the 

 surface. 



-N'ot only do tlic anionnt of wa.tci' and the ilc]ilb of the 

 water level help lo (b'(ermin(^ plant societies, but also the 

 substances which the water (Mjnlaiiis. Two ai'ea.s mav have 



