128 AERATION AND AIR-CONTENT. 



Gates (1914 : 483) concludes that the xerophytism of the ever- 

 green ericads is real, and that it has been brought about primarily 

 by winter conditions. The transpiration is lower the more xerophytic 

 the structure of the leaves, and this is greatest in the plants most 

 exposed to winter conditions. While the xerophytic structure also 

 reduces water demands in the summer, it seems to be unnecessary 

 then, for neither extreme drought in the field nor the extreme evapora- 

 ting conditions of a laboratory injured the many plants of Chamce- 

 daphne used. On the other hand, thousands of plants were killed 

 down to the snow-line during the continued severe dry winter-cold of 

 1911-12, though the average conditions of the preceding winter did 

 not have the slightest effect. During the winter the transpiration 

 and rate of conduction were much higher in the evergreens than in 

 deciduous plants, while in the summer they were much higher in the 

 herbs and deciduous woody plants than in the evergreens. 



Dachnowski (1912) has brought together all of his previous results 

 in one comprehensive account, and reaffirms his conclusions as to the 

 fundamental role of toxicity and the paramount importance of 

 bacteria in producing it. He emphasizes the inadequacy of lack of 

 aeration and low soil-temperatures as causes of xeromorphy, but 

 assigns much importance to the presence of oxygen in decreasing 

 toxicity. The behavior of the roots of bog xerophytes is said not to 

 be due to low oxygen-content, but the growth is inhibited by the re- 

 ducing action of the substratum and the incomplete disintegration 

 of organic compounds, conclusions that are not supported by the 

 facts of anaerobic respiration. The quantity of the products of 

 bacterial decomposition are thought to constitute a toxic, physio- 

 logically arid habitat at one stage and an available supply of plant 

 nutrients at another stage of the process, and hence acidity, toxicity, 

 and reducing action represent merely a stage in the decomposition of 

 organic matter. 



Rigg (1913 : 325) has found that Tradescantia shows stunted 

 root-hairs when grown in bog-water, but develops normal ones in 

 lake and spring water, as well as in water from drained or partly 

 drained bogs. The stunting effect disappears when bog-water is 

 diluted with an equal volume or even one-half of its volume of tap- 

 water, and it is increased by boiling the water down to a fraction of 

 its original amount. All of this is in harmony with the fact that 

 many typical bog-plants have no root-hairs. The toxin or toxins in 

 bog-water lose their effect with drainage. It is thought possible 

 that this toxin prevents mesophytes from growing in bogs by reducing 

 the amount of absorptive root surface. It is obvious that practically 

 all the effects observed can be correlated with the absence of oxygen. 

 The author's statement that the mere absence of air from water 

 does not render it toxic must be completely revised, if toxicity is to 

 be measured by injury and death. In a later paper it is shown that 



