282 KEY. PKOF. &. HENSLOW ON THE [Aug. 1907, 



15. On the Xeeophttic Chaeactees of certain Coal-plants, and a 

 Suggested Oeigin of Coal-Beds. By the Rev. Prof. Geoege 

 Henslow, M.A., F.L.S., F.G.S. (Read May 1st, 1907.) 



In endeavouring to interpret the nature of the coal-flora, we can 

 only do so by comparing the various members of it with living 

 plants ; and, when this is done, their anatomical structures exhibit 

 both hygrophytic as well as strong xerophytic characters in various 

 genera. It has been generally supposed that the whole of the 

 coal-flora consisted of marsh-herbs and trees. They all, therefore, 

 ought to reveal hygrophytic characters, such as the following, which 

 are nearly always to be found in living aquatic plants : — 



If they be hydrophytes, then there will be, as a rule, no axial 

 root. An. illustration of the basal part of a stem of a Calamite, in 

 Prof. Seward's work on « Fossil Plants ' vol. i (1898) p. 343, affords 

 an excellent example of an aquatic plant. The arrest of the axial 

 root and lower extremity of the stem is probably due to the absence 

 of oxygen in the mud necessary for respiration, and the presence of 

 poisonous gases, such as marsh-gas. A similar form of stem with 

 adventitious roots is well seen in our common (Enanthe Phellandrium. 

 This is, in fact, characteristic of Dicotyledonous aquatic plants and 

 of all Monocotyledons. The form of the stem is an inverted cone, 

 having adventitious roots, forming the so-called ' prop-roots ' of 

 mangroves (and of the terrestrial Pandanus or screw-pine, maize, 

 and palms, among Monocotyledons) ; or else the presence of ' pneu- 

 matophores' is the most characteristic morphological feature of 

 water- or swamp-plants. 



With regard to the internal tissues, there will be an abundance 

 of lysigenous lacunae, which may occur in the pith, in the inner 

 extremity of the vascular bundles, and in the cortex of the roots. 

 Regularly - formed schizogenous lacunae, constituting complete 

 aerenchymous tissue, may be looked for in the stems and petioles 

 of leaves. On the other hand, a relatively-great reduction of 

 supporting tissues will occur, with no sclerenchyma in the sub- 

 merged organs, although this tissue may be necessary in the aerial 

 portions, if the atmosphere be dry. 



The great and uniformly deteriorating effect of water upon 

 all kinds of vegetation must not be forgotton, when we are en- 

 deavouring to account for the abundance and gigantic size of many 

 of the trees of the Coal Period. 



Commencing with the Equisetales — represented now by the 

 sole genus Equisetum or horsetail — the members appear to have 

 been upon the whole decidedly hygrophytic ; though this does not 

 preclude the fact that some species may have become adapted to 

 live on dry land, and acquired characters of a more or less xero- 

 phytic nature in adaptation to new environments. Such, appears to 

 have been the case with Calamodendron intermedium, which has 

 strengthened its stem by making part of the medullary rays 



