120 AERATION AND AIR-CONTENT. 



Larix laricina, Cornus stolonifera, Osmunda regalis, 0. cinnamomea, 

 Rhus vernix, and Aster junceus. 



Rayner (1913, 1915) has shown that when seeds of Calluna vulgaris 

 are sterilized and germinated in sterile conditions, root-growth is 

 retarded and finally inhibited in the absence of mycorrhizal infection. 

 Pot cultures in soils favorable and unfavorable to the growth of the 

 plant in the field demonstrated that Calluna grew normally in the one 

 and abnormally in the other, as indicated by poor germination, the 

 stopping of growth in root and shoot, and the presence of bacterial 

 colonies on the roots about the tips. The relation between the plant 

 and its mycorrhiza seems to be obligate, and normal growth is de- 

 pendent upon early infection and the healthy growth of the fungus. 

 The usual preference for an acid soil is explained by the fact that lime 

 prevents the normal development of the fungus and promotes the 

 growth of the colonies of bacteria, interfering with the symbiotic re- 

 lations of the root and its proper functioning. Further studies of the 

 fungus showed that it is not confined to the roots, but is found also in 

 stems, leaf, flower, and fruit. Seedlings free from infection did not 

 form roots, but underwent complete cessation of growth, remaining 

 alive but rootless for months. It is regarded as possible that the 

 presence of the fungus in stem and leaf permits it to fix atmospheric 

 nitrogen. 



Gates (1914 : 472) has shown that the summer transpiration of 

 Chamcedaphne is much less than that of such hydrophytes as Sagit- 

 taria latifolia and Carex filiformis, while Andromeda transpires nearly 

 as much as Potentilla palustris and somewhat more than Aspidium 

 thelypteris. The rate of conduction in the evergreen heaths was found 

 to be much lower than that of the hydrophytes. The latter exhibited 

 the highest rate of water-loss per unit area, and in general herbs 

 transpired more rapidly than shrubs. The more hydrophytic swamp- 

 shrubs transpired more vigorously than the typical bog-shrubs, and 

 the deciduous more than the evergreen. Water-loss in the deciduous 

 Larix laricina was noticeably greater than in the deciduous broad- 

 leaved Acer rubrum, and decidedly higher than for the evergreen 

 conifer, Picea mariana. The transpiration of the evergreen shrubs 

 was several to many times greater than that of the deciduous shrubs 

 during the winter, under both outdoor and indoor conditions. The 

 author states that some so-called xerophytic plants use as much or 

 more water than ordinary mesophytic plants, but they are xero- 

 phytic because they can not absorb a large amount of water in 

 proportion to that which they would otherwise transpire. This is 

 puzzling, and is in opposition to the later statement that the deter- 

 mination of the rate of transpiration per unit area of leaf-surface by 

 weighing is a satisfactory approach to a knowledge of the demands of 

 plants for water. 



