304 JOURNAL OF FORESTRY 



italics. The error is not original to Waller ; it goes back a great many 

 years. Nevertheless we feel bound to controvert it whenever and 

 wherever it appears. Briefly, our idea is : 



All successions are toward greater xerophytism, if we use this word 

 to describe the low water requirements of the plant. They begin 

 either in free water or in very young soils. In an undrained glacial 

 bog, the succession from aquatic plants to the xerophytic spruce may 

 be almost direct, because of the high concentration of the stagnant 

 water. From fresh water and from fresh soils as a beginning, we 

 may have many more stages before xerophytism is reached. Free 

 drainage in a young soil keeps the soil solution always at a low osmotic 

 pressure. Aging through topographic change reduces drainage, and 

 simultaneously the constant addition of humus increases the solutes in 

 the soil and raises the osmotic pressure. In the final struggle of a 

 seedling, the success of which determines the character of the plant 

 formation, the quantity of water available is practically not a factor. 

 The success depends on whether or not the plant exerts a sufficiently 

 strong osmotic pressure to absorb the water, and possesses a correspond- 

 ingly economical transpiration rate. In the later stages of succession, 

 when competition for light is keen, the struggle is likely to go to the 

 plant which is most efficient in photosynthesis. The shade-endufing 

 species withstands the greatest degree of drought. The terms as here 

 used have, of course, no application to the desert climax, in which the 

 static condition has produced specialized plants of a stagnant, fre- 

 quently dormant, character. 



While then, the advanced stages of succession do ordinarily produce 

 a soil of greater moisture-holding capacity, this merely encourages the 

 initiation of a greater number of individuals and a keener competi- 

 tion for the moisture and light. The character of the plant formation 

 can not be in any sense the result of mass-action. It is absolutely a 

 struggle of individuals, which is probably more keen between indi- 

 viduals of the same species than it ever is in a mixed association. The 

 idea which I wish to convey is that we lose the ecological significance 

 of the facts when we give weight to the statement that the plants of 

 the climax formation, in the aggregate, require more water than the 

 earlier stages. We lose sight of the fact that the individual must be 

 equipped for a very keen struggle both for moisture and light. 



In the discussion of climatic factors a number of good points are 

 brought out by Waller, among which might be mentioned the idea 

 that it is the extremes of heat and cold, not the mean temperatures, 

 which kill plants and hence are important in distribution ; as to light, 

 in its broad distribution, it is asserted that it is rarely a limiting factor. 

 We wonder, however, if this may not be controverted by the recent 



