284 REV. PROF. Gt. HENSLOW ON THE [Aug. I907, 



Ijet us, then, see what existing facts might be thought to support 

 the usual idea of all the trees of the Coal-Measures growing in 

 swamps. It is not to swamps of cooler latitudes, in which trees do 

 not grow, but to tropical swamp -forests that we must turn for them. 

 Dr. Schimper tells us of some in which none but palms nourish. 

 As palms were undoubtedly descended from aquatic ancestors, 

 their return to hygrophytism does not surprise me. But, in other 

 swamp-forests, as in Louisiana and Burma, the trees represent 

 genera of certain families of Dicotyledons which have become 

 adapted to water by developing aerenchyma in the place of cork at 

 the bases of the trunks, these having expanded into a conical form 

 for the purpose. 1 



Adaptations have occurred with all of our own aquatic plants,, 

 which consist of individual species or genera of terrestrial families, 

 such as the water-crowfoot among buttercups, the water-violet 

 among primroses, the beckbean among gentians, etc. Taking, 

 however, the Cordaites, for example, as a group, and comparing 

 them with existing Cycads or other Gymnosperms, not only are the 

 latter very characteristic xerophytes, but structural affinities point 

 to the former as having been xerophytes also. On the other hand,, 

 neither group exhibits any hygrophytic structures whatever. 



With regard to the suggestion that mangroves and plants of 

 maritime saline areas might supply parallels to some coal-plants, 

 it is not enough to observe that they have certain xerophytic 

 characters, such as a water-storage tissue in the leaves of the former 

 and succulency in the case of the latter, but that they possess far 

 more characteristic hydrophytic features. It is in these features 

 that we find no parallels or resemblances to them in trees of the Coal- 

 Measures. For example, in the marine mangroves there is a total 

 arrest of the primary axial root, so that the trunk is supported on 

 ' prop-roots.' Secondly, as aeration of the roots is always difficult 

 in marshes and in the mud below water, the roots and basal parts 

 of the trunks are provided with pneumatophores, or else aerenchyma, 

 respectively, of most marked characters ; as is the case with our 

 marsh-samphire (Salicornia herhacea) covering our salt-marshes. 

 Nothing of the sort appears to be known in Stigmarias, for no such 

 characters have been described. Moreover, areas swept by tides 

 twice a day, whether they be tidal mangrove-swamps or maritime 

 salt-marshes, must be more or less incapable of accumulating much 

 vegetable-matter like peat-bogs or inland forests. 



In addition, it may be observed that the presence of freshwater 

 shells and terrestrial insects in the Coal-Measures does not lend 

 much (if any) countenance to littoral trees forming, at least, any 

 large proportion of coal, even if it were possible. Oysters cling to 

 the roots, while crabs run on the mud around mangroves. 



With regard to the Perns, they appear to have much resemblance 



1 A. F. W. Schimper, 'Plant-Geography' [transl. Fisher] 1903, fig. 392, 

 p. 652. See also a paper by M. C.-L. Gatin, ' Observations sur l'Appareil 

 respiratoire des Organes souterrains des Palmiers ' Revue Generale de Bota- 

 nique, vol. xix (1907) p. 193. 



