40 Plant-life of the Oxford District 



such rhizomatous forms, once the rhizome-habit has been attained by the 

 loss of the normal factors of arboreal growth in the main stem, the capacity 

 for geotropic erection and internodal elongation may be retained to a certain 

 extent, while the orientation and display of the foliage laminae may be still 

 the prerogative of the leaf-petioles. 



I. Hence the First Phase of the Obligate Aquatic x is that of a herba- 

 ceous plant, with rhizome rooted in the mud of a woodland swamp-area, 

 sending up erect leafy axes subsequently to produce flowers and fruits in 

 free air, in the normal manner of woodland undergrowth, but already charac- 

 terized by a feebly-absorbing root-system. In damp air the shoot-system 

 may retain fairly normal construction ; but in more open situations xero- 

 morphic features begin to be shown, as the leaves tend to lose water faster 

 than they can take it up ; and shoots with greatly reduced foliage-members 

 become characteristic. In this respect it is interesting to compare the 

 limiting reduction of Equisetum (Horsetail) of the Pteridophyta, with its 

 leaves reduced to merest points, and photosynthesis entirely restricted to 

 the green shoots, with the Angiosperm Hippuris (Mare's Tail), the latter with 

 greatly reduced leaves on the subaerial shoots, and minute inconspicuous 

 flowers : again as expressing the fact that progression in aquatic habit is 

 by no means a factor of time. A certain level of attainment, once estab- 

 lished, may be so maintained indefinitely and unchanged, so long as the plant 

 may so come to occupy an 'inferior' station in which there is little competition. 

 Equisetum, as the typical aquatic of this category, is at the same time one 

 of the oldest known types of Land-flora, as a greatly deteriorated representa- 

 tive of a race that was arboreal as Calamites of the Palaeozoic, and probably 

 beyond : Hippuris, a seed-plant of the more modern epoch, does not get so 

 far in shoot-reduction and specialization, though curiously similar in general 

 dimensions and whorled habit, but other Angiosperms go far further. 



II. So long as the shoot-system can elongate above the water-level, 

 and the roots can obtain free oxygen from the over- water parts, no specially 

 new problem arises ; but where the stem remains wholly submerged, reduced 

 to a prostrate dorsiventral rhizome, or bearing terminal rosettes of leaves, 

 the entire onus of the erection and display of the foliage members in the 

 air, falls on the leaf-petioles and the leaf-lamina itself. Where again the 

 rhizomatous plants are closely gregarious, occupying all the substratum 

 available, lateral extension will be precluded, vertical elongation is the only 

 solution, and competition sets in for the production of a new close-canopy 

 of elongated leaf-members, giving the characteristic * Rush-habit ' (Spar- 

 gamum? Acorus, Iris, Butomus). In such case the * spearing ' habit of the 

 young leaves to reach the surface is enormously exaggerated ; and in the 

 production of an erect linear leaf-member the distinction between petiole 

 and lamina may practically disappear. 3 



1 The case of the Aquatic requires more extended notice, not only because aquatic vegetation is 

 particularly well displayed in the local flora of swamp-woodland and river-system ; but while the 

 shrub and the herbaceous types of the woodland show with sufficient clearness their reduction from 

 the arboreal habit, the general effect of much of the aquatic vegetation (especially Monocotyledonous 

 forms) is only remotely suggestive of a tree-organization. Resemblances are traced in details of 

 anatomy rather than in general morphology. A sequence of types is therefore arranged to illustrate 

 the more complete adaptation to an aquatic environment secondarily, in correlation with progressive 

 loss of original arboreal mechanism. To read the story the other way, as a rise of Angiosperms to 

 the land from comparable modern aquatics, involves some account of the manner in which such 

 structural features as main axes (cambium, timber) could have been evolved from originally wholly 

 submerged aquatics. 



2 Sparganium ramosum is the typical ' Rush ' of Rush baskets, Rush mats, and Rush-seated 

 chairs, as a valued economic plant of older times. Fragrant Rushes for strewing on floors were 

 Acorus Calamus (leaves), and the Rushes of Rushlights Juncus effusus, flowering axes. 



3 Schimper (1903), Plant Geography, p. 810, Ssoetes-type. Arber (1918), The Phyllode Theory 

 of the Monocotyledonous Leaf, Annals of Botany, xxxii, p. 465. 



