WATER REQUIREMENT AND ADAPTATION IN EQUISETUM 43 



range in the amount of water that plants can take from the soil 

 is greater than indicated by the work of Briggs and Shantz. 



4. The fact that Equisetum can adjust itself to physical 

 changes in the habitat by marked physiological responses but 

 gives negative results as to adaptation indicates that the xero- 

 morphic features are extremely stable structures and that 

 adaptation to hydrophytic conditions has come about through 

 the development of air-spaces, diaphragms, and permanently 

 open stomata. 



5. The great amount of water lost through transpiration and 

 guttation indicates that this species does not grow in the 

 xerophytic regions of bogs because of its low absorptive powers, 

 as Dachnowski maintains, and that it cannot be considered an 

 actual xerophyte at all. Its xerophily is only superficial and is 

 probably due to the persistence of ancestral structures, now 

 compensated by adjustment and by adaptation in the aerenchym. 



6. Since the structure and habitat of the Paleozoic Catamites 

 and Mesozoic Equisetum were essentially the same as those of 

 the present day form, the apparent xerophily cannot be con- 

 sidered as an indication of a dry climate, of a physiologically arid 

 habitat, because the water lost by the plants is great and means 

 of checking it are absent. It is probable that the superficial 

 xeromorphy of Equisetum was acquired in a cold arid period, 

 such as the Permian. 



7. The hydrophytic nature of Equisetum fluviatile places it in 

 the hydrarch succession. It usually plays only a secondary 

 part in the various kinds of swamps where it probably holds its 

 place as a relic of former eoseres. Successionally it is a member 

 of the reed-swamp or sedge-swamp associes, usually forming a 

 socies. 



8. Equisetum hyemale and Equisetum arvense are found in 

 much dryer areas than Equisetum fluviatile. A comparison of 

 their transpiration per unit area shows that the former are more 

 mesophytic in their tendencies. This accounts for the fact that 

 these two species are capable of invading grass land. 



