118 AERATION AND AIR-CONTENT. 



Sampson and Allen (1909 : 49) studied the transpiration of Scirpus 

 lacusiris in comparison with that of Helianthus annuus, a pronounced 

 mesophyte, and found that the former lost water almost twice as 

 rapidly. Dosdall (1919 : 35) has shown that the transpiration of 

 Equisetum fluviatile is twice that of Helianthus annuus, and Clements 

 and Goldsmith (1921) have recently found that Typha transpires 

 several times as rapidly as the sunflower. 



Transeau (1905 : 17) has studied the structure of a number of 

 representative bog-plants or bog-forms, namely, Eriophorum vir- 

 ginicum, Sarracenia purpurea, Oxy coccus macrocarpus, Andromeda 

 polifolia, Chamcedaphne calyculata, Chiogenes hispidula, Vaccinium 

 corymhosum, Salix sericea, Ledum grcenlandicum, Larix laricina, 

 Picea jnariana, and Pinus strobus. In general, they are character- 

 ized by a thick cuticle, waxy coatings and hairs, thick-walled epi- 

 dermal and hypodermal tissue, the presence of palisade tissue, and of 

 resinous bodies in the roots and leaves. There is a general reduction 

 in the size of the leaves, which are often revolute, and mycorrhizal 

 fungi are present in the roots of most. They resemble the xerophytes 

 of dry sandy plains in the reduced leaves, epidermal characters, and 

 palisade tissue, but differ greatly in root development and structure. 

 All of the woody plants listed possessed mycorrhiza, with the excep- 

 tion of Andromeda, Chamcedaphne, and Salix, and it was observed 

 Hkewise in Betula lutea, B. pumila, Oxycoccus oxycoccus, and Populus 

 tremuloides. In the case of Larix, it was found that mycorrhiza 

 developed only in poorly aerated substrata and that plants formed 

 normal roots under aeration, or in a soil naturally well-aerated. It 

 was concluded that acidity had nothing to do with the production of 

 mycorrhiza, since normal roots were developed in acid water-cultures. 



Yapp (1909 : 309) has shown that there is a marked difference in 

 the evaporation at different levels in a marsh community. In vege- 

 tation 5 feet high, the evaporation was 100 just above, 32.8 in the 

 middle, and 6.6 at the soil level, while in that 2 feet high it was 100 

 just above, 56.2 just below, and 14.7 at the bottom. He concludes 

 that the mutual protection from excessive transpiration and the 

 mechanical effects of the wind, derived from the grouping of shoots, is 

 probably beneficial, even apart from the more obvious cases where the 

 chmate is exceptionally rigorous. The structural features of vege- 

 tation may thus be effectual in securing immunity from excessive 

 transpiration. Different species vary as to the depth of the root- 

 system, height of shoots, relative position of transpiring surface, and 

 length of vegetative period. Few of the species in a swamp-moor live 

 under precisely the same set of physical conditions. Thus, the argu- 

 ments of authors, who insist that the so-called xerophytic structures 

 of marsh-plants are not due to present-day conditions, because both 

 xerophytes and non-xerophytes often grow side by side, are entirely 

 inconclusive. However, this last statement would doubtless have 



