i 9 o6] TRANSEAUBOGS OF THE HURON RIVER VALLEY 21 



dan distorted, resin bodies throughout cortex and meristematic 

 tissues of the wood. 



To summarize these characteristics, it is evident (i) that epidermal 

 and hypodermal tissues are thick- walled ; (2) that for the conserva- 

 tion of water these are reinforced outwardly by a heavy cuticle, by 

 coverings of wax and air containing hairs; (3) that resinous bodies are 

 found in the roots and leaves of many of the plants; (4) that there 

 is a general reduction in the size of the leaves, and that these are 

 frequently revolute-margined ; (5) that palisade tissue is quite uni- 

 formly developed ; (6) that mycorhizal fungi are present in the 

 roots of most of the plants; (7) that, when compared with the xeror 

 phytes of dry sand plains (25, 6), they show a similarity in respect 

 to the reduction in size of the foliage, in the development of external 

 protective coverings of the sub-aerial parts, and in the presence of 

 palisade tissues, but are very different in the matter of root develop 1 

 ment and character of root structures. 



To account for the peculiarities of the bog vegetation various 

 theories have been brought forward. KIHLMAN (28), in accounting 

 for the xerophilous character of the plants of arctic swamps, which 

 include several species common to American bogs, lays stress upon 

 two factors: (i) the low temperature of the moist substratum, and 

 (2) the presence of drying winds. The former influences the plants 

 by decreasing the power of absorption, the latter increases the rate 

 of transpiration. The plants of such habitats must therefore be 

 protected against the loss of water by the subaerial parts. 



SCHIMPER (44, p. n) in classifying the natural habitats in which 

 xerophytes occur mentions among others "peat bogs, because of the 

 humous acids in the soil." On page 18 he says: 



The xerophilous character of the vegetation of peat moors has hitherto been 

 considered an incomprehensible anomaly, and yet the rich supply of humous 

 acids in the soil furnishes a condition for its occurrence as comprehensible as it 

 is necessary. The presence of Scotch pine and heather on both dry sand and on 

 wet peat is thus not more remarkable than is that of Ledum palustre, Vaccinium 

 uliginosum, and other peat-plants on the cold dry soil in the polar zones. 



Further (p. 124) the statement occurs that "on the very acid humus 

 of moors the vegetation assumes a decidedly xerophilous character, 

 because the humous acids impede the absorption of water by the 



