RECENT ADVANCES IN SCIENCE 641 



maximum for the steppe plants and a minimum for the swamp 

 plants. In the same plant the different parts were found to 

 have an osmotic pressure that varied inversely with the 

 moisture of their respective environments. Even in one and 

 the same leaf it was found that provided the blade were broad 

 the osmotic pressure near the margin was sensibly higher 

 than near the midrib. 



The conclusion seems warranted that osmotic pressure is 

 dependent upon the humidity of the environment whether con- 

 ditioned by external influences or internal structure. 



Several of the different types of woodland in Britain have 

 now been described more or less fully for considerable areas, 

 and to these must now be added the oakwoods (Quercus Robur) 

 with hornbeam undergrowth as represented in Hertfordshire 

 (Salisbury, Journal of Ecology, vol. iv. pp. 83-117). These 

 are associated with the lighter clays and loams of the clay- 

 with-flints and the same type occurs on similar soils in Essex, 

 Kent, and Middlesex. It seems to be a quite natural type not 

 met with on the heavier clays where the undergrowth is hazel. 

 In the herbaceous vegetation several societies can be recog- 

 nised, marked by the abundance of either bracken, wood 

 anenome, dog's mercury, lesser celandine, or creeping buttercup. 

 The soil water is least where the bracken flourishes and increases 

 in the other societies in the order named. Dog's mercury, 

 however, sometimes abounds in dry situations where the 

 acidity is low. The evidence available would appear to 

 indicate that the diffuse light intensity in woodlands is 

 always the limiting factor to assimilation. Two phases can 

 be recognised depending upon the illumination, viz. a light 

 phase (from leaf fall in autumn to leaf expansion in spring) 

 and a shade phase. During the former the light intensity is 

 from 30 per cent, to 40 per cent, of that outside the wood, whilst 

 during the latter it falls to from 1*3 per cent, to o*i6 per cent. 

 The biological relations of the herbs are intimately bound up 

 with this diminution, a fact strikingly illustrated in their 

 flowering periods and the duration of their foliage. The changes 

 consequent upon coppicing the shrub layer are very pronounced 

 and mainly related to the great increase in illumination. For 

 example the number of species in the interior of the wood 

 before coppicing may be only eleven, whilst two years after 

 removal of the shrub layer the number may be over sixty, 

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