Vol. 63.] XEROPHYTIC CHARACTERS OE COAL-PLANTS. 283 



sclerenchymatous. A parallel case is seen in our common species 

 Equisetum arvense, which, although retaining its hereditary lacunas, 

 has run up silicified x ribs on the circumference of the stem for 

 the sake of strength. 



It has been suggested that, since several herbaceous plants 

 occur in the humus of bogs in moors, having xerophytic features, 

 the trees of the Coal-Flora might have lived in freshwater marshes 

 notwithstanding their pronounced xerophytic or mesophytic 

 characters. But, since fresh water never gives rise to 

 xerophytic structures, it must be observed that the presence 

 of such plants in bogs is due to migration from dry situations ; 

 they have, then, retained such ancestral morphological characters as 

 are immaterial, provided the internal structure of the roots has 

 become adapted, if necessary, to aquatic conditions. This is what 

 takes place, as seen in the aerenchyma of the pneumatophores even 

 of strongly-xerophytic trees, as notably in the deciduous Cypress 

 (Taxodium distichum). As examples of plants normally found in 

 dry woods, heaths, etc., Dr. A. F. W. Schimper especially notes a 

 grass (Aira flexuosa) and a parasite (Melampyrum pvatense) as 

 growing in bogs. 2 



As some stress has been laid upon the presence of xerophytic 

 plants in bogs as supporting the idea that coal-plants were natives 

 of marshes, it will be as well to quote Sir J. D. Hooker's remarks 

 on the above grass, as he gives the usual and natural habitat 

 ('Student's Flora' 3rd ed. 1884, p. 483):— 



' Dry woods, heaths, etc., N. to Shetland, ascends to 3700 feet in the High- 

 lands Leaves short, curved, grooved.' 



Such are xerophytic features, and no mention is made of their 

 presence in bogs. Hence it would be somewhat misleading to call 

 these interlopers ' bog-plants.' What really takes place is, I re- 

 peat, that the form or external morphological features 

 are retained, but (wherever necessary) the anatomy is 

 cha nged. Hence such plants might be regarded as having become 

 xeromorphic hygrophytes ; just as when the water-crowfoot grows 

 on the mud of a dried-up pond it becomes a hydromorphic mesophyte. 

 But, in the case of Lejndodendra, these do not seem to have any 

 signs of specially-adaptive tissues in harmony with water. The 

 roots (Stigmaria) and trunks are xerophytic, as Prof. Seward has 

 pointed out. They could only have acquired their characters by 

 iaving lived for many generations from their first origin on dry 

 ground, even if they ever migrated to a swamp and retained their 

 characters. 



1 According to MM. Gaston Bonnier & Leelerc du Sablou, 'Cours de Bota- 

 nique'vol. ii (1904) p. 1343, who describe them as fibres silicifiees de 

 l'ecorce. 



2 Dr. Schimper's suggestion that the cause of xerophytes in bogs may be 

 due to humic acid has been disputed, if not disproved ; nor does he state why 

 other and normal bog-plants growing with certain xerophytic types remain 

 hygrophytes, 'Plant-Geography' [trans! . Fisher] 1903, p. 111. See also 

 Clements's 'Eesearch-Methods of Ecology ' 1905, p. 126. 



