66 FLOWERS OF THE HEATHS AND MOORS 



medium supply of moisture, that do not grow in dry places, with the 

 broadest leaves, which is not what one would expect. Galium palustre 

 has narrower leaves than G. Aparine, a mesophilous species. More- 

 over, some heath plants can grow on dry warm soil and on cold wet 

 soil, as Ling. Crowberry, &c. This suggests that there is some corre- 

 spondence between the two types of soil, and that some of the factors 

 of life in the case of marsh plants necessitate an economy in the use 

 of water. There is a transpiration optimum, and marsh plants may 

 have to depress their transpiration. 



A wet soil is cold, the roots can absorb no water if the temperature 

 of the soil sinks below a certain degree, and the soil is thus what is 

 called physiologically dry. This is seen in the later character of heath 

 and moor or marsh plants, and in the fact that the flowers are in bloom 

 much later than on dry soils. The plants are clothed with hairs, there- 

 fore, to prevent an excess of transpiration over absorption, and this is 

 accelerated by high winds. Respiration is affected owing to the soil 

 being badly aerated or lacking in oxygen, so that the activity of the 

 aerial parts must be the less, owing to the less amount of oxygen 

 absorbed by the roots by marsh plants. Hence the ability of Heaths 

 that grow on heaths and dry warm soil to grow on moors, as a heath is 

 just as badly aerated, with often periodically soaked raw humus or dry 

 peat. Peat retains water more than other soils except clay. The cause 

 of physiological drought here may also be clue to the abundance of 

 humous acids, &c., in moor soil or peat, which affect the roots and pre- 

 vent or deter absorption, so that the plants wilt if transpiration is rapid. 



Though many plants of heaths and moors exhibit the above xero- 

 philous characteristics, others are hydrophilous in character, and so 

 are the adaptations to which they give rise in the flora. 



Transitional from marsh associations is the low-moor formation, 

 which is characterized by humous acids in the soil containing vegetable 

 accumulations, forming peat with Reeds, with much nitrogen. The 

 water contains calcium and potassium compounds, and is thus like a 

 marsh, often enclosing a marsh or adjoining it. 



Sedges grow in tufts, giving rise to sedgemoor, and other plants 

 are Cotton Grass, Rushes, Arrowhead, Helleborine, Angelica, Bog- 

 bean, Marsh Bedstraw, Marsh Willowherb, Grass of Parnassus, and 

 Willow, Birch, Alder, Heaths, &c. 



Several associations can be recognized, as amblystegiata (from a 

 moss), cariceta, eriophoreta, molinieta, junceta, &c. There are usually 

 herbaceous perennials and a ground flora of mosses forming two layers 

 below the former. Few are ligneous, and only a few are annuals, as 



