90 REACTIONS. 



total amount present; so much so that even cultivation may affect no appre- 

 ciable reduction in 50 years, as shown by the experiments at Rothamsted 

 (Hall, 1905 : 36). In addition, all the nutrients absorbed are returned sooner 

 or later, and in most communities the annual retiun must nearly counter- 

 balance the use. In any event, there is no indication at present that succes- 

 sional movement is affected by the direct decrease of nutrients through 

 absorption 



The formation of heath-sand or "bleisand" probably furnishes an example 

 of reduction in nutrient-content as a consequence of another plant reaction. 

 This is the formation of acids by humus. These render the mineral nutrients 

 soluble, and the latter are then removed by the percolating water, beginning 

 at the top. In extreme cases, httle remains but quartz sand, which acquires 

 a characteristic leaden color in consequence of the precipitation of particles 

 of humus. Such "bleisand" represents in consequence the extreme of poverty 

 in regard to soil nutrients. It makes the ecesis of more exacting species 

 almost impossible, and thus secures the persistence of the heath stage for 

 very long periods, so that it may often be regarded as a climax. 



(14) Reaction by producing adds. — ^The direct reaction of plants in excret- 

 ing carbon dioxid from the root surface has already been considered under 

 "Weathering." It is probable that this bears no relation to the production 

 of acids in the more or less partial decomposition of humus. Wherever plant 

 remains accumulate abimdantly in water or moist places, access of oxygen is 

 difficult. The decomposition is slow and partial, and the water or soil becomes 

 more or less acid. The acids formed are very httle imderstood, and the process 

 by which they are formed is likewise obscure. Lack of oxygen seems a neces- 

 sary condition of their production, and the effect of the acid upon plant growth 

 is complicated with the effect of deficient aeration. Both, apparently, act 

 together in diminishing the absorptive power of roots, probably in consequence 

 of decreased respiration. This apparently places a premium upon plants 

 with modifications for reducing transpiration, and acid areas are usually 

 characterized by so-called "bog xerophytes" such as Ledum, Kalmia, Vac- 

 dnium, etc. In spite of much recent study, the nature of bog plants is still 

 an open question. It seems increasingly evident that most of the xeroid 

 species of wet places are not xerophytic at all, but that a restricted group 

 characteristic of peat-bogs, heath-moors, etc., are actual xerophytes. Even 

 with these, however, no final solution is possible until their water requirements 

 have been studied experimentally and their transpiration response is known. 

 In so far as succession is concerned, the production of acid in swamps modifies 

 the normal reaction of decreasing water-content, and marks a series of stages 

 which dominate for a time, owing to a favorable response to poor aeration. 

 Whenever the latter is improved by drainage, filling, or a drier climate, condi- 

 tions become more favorable to species of neutral or alkaline soils, and the bog 

 plants disappear in consequence or as the outcome of competition. "The work 

 of Gates (1914) confirms the assumption that the bog heaths are the result of 

 winter xerophily, while a recent study of the transpiration and growth of 

 plants in aerated bog-water indicates that the acid is a concomitant only, and 

 not a cause (plate 25 a). 



(15) Reaction by producing toxins. — ^The question of the direct production 

 of toxic substances by excreting plant roots is a much mooted question. 



