Subordinate and Herbaceous Flora 39 



able for their existence. This same faculty for lateral progression has again 

 a wide application in the attainment of a gregarious habit; e. g. ( in cover- 

 ing soil with a mantle of protective vegetation on the part of xerophytes, 

 or in travelling towards the source of water in facultative aquatics. All 

 aquatics have apparently passed through this phase. In all cases the 

 gregarious habit initiates a new form of canopy, preferably indicated as 

 a ' mantle ' or ' mat '-formation, and the individual organisms acquire 

 a certain amount of control of their special environment by collective action, 

 which again puts them on a higher plane. 1 



The case of the Aquatic is more complex, since the water-problem so 

 insistent in the general case of the land-plant is largely eliminated ; perenna- 

 tion is restricted to periods of winter-cold and darkness, only in the limit to the 

 possibility of the total disappearance of the water. But even in the growing 

 season temperature is largely regulated by that of the water, which shows 

 relatively little range, and may be widely different from that of the air ; while 

 the aeration problem of the submerged portion becomes critical, since the free 

 oxygen-supply is at best only \ per cent, by volume, as compared with the 

 30 per cent, of the free atmosphere. No aquatic can make good in the 

 water until it has solved the problem of aerating the submerged root-system 

 by which it absorbs its food-salts, by means of an exaggeration of the 

 system of intercellular spaces, and the retention of the waste O 2 of photo- 

 synthesis. As this method is the best thing the plant can do, 2 but is never 

 wholly a success, the root-system remains permanently handicapped. Its 

 absorptive capacity is strictly limited, with consequent result on the reduced 

 transpiration system, as exhibited in the further deterioration of all vascular 

 tissue, and restricting the supply of essential food-salts. Water-plants 

 become characteristically starved, pulpy and parenchymatous, with lacunar 

 tissue, and little mechanical efficiency beyond the maintenance of the 

 turgidity of the active cell-units with abundant and cheap water-content. 



On the other hand, since protoplasm is itself a medium containing 

 over 90 per cent, water, it is evident that while no effective plant-life can 

 ever flourish under extreme xerophytic conditions, however intensive it 

 may be during its short spell of active growth, the aquatic environment is 

 clearly the original one for all plant-life, and such regression to the water 

 may open up new possibilities. Since the water-problem, which is after all 

 the most difficult proposition for all land-flora, is now largely discounted, so 

 long as the water itself is available, it remains only a matter of difficulty 

 for the parts which are out of the medium ; and such plants return to the 

 water armed with their age-long experience as seed-plants of the land, and 

 are now on a new footing in their old environment. 



The case of the Facultative Aquatic, as that of a rhizomatous plant rooted 

 in the ground, and following failing water-supply, enduring submergence, 

 or evm floating on the surface, but without any special anatomical adapta- 

 tions for aeration, remains wholly incidental. The story of the Obligate 

 Aquatic begins with the plant rooted in subaqueous ground, and supplying 

 oxygen to its root-system from the over-water green shoots. To such 

 a plant the amount of water present begins to be a secondary consideration. 

 The vegetative parts grow above the surface to reach air and light, and the 

 flowering shoots may retain much of their original erect branching habit, 

 with flowers of primitive or even highly elaborated organization, on a par 

 with that of the xerophyte of a short season, or of actual forest-trees. In 



1 For such possibilities of invasive action by gregarious rhizomatous forms residual from types 

 of tropical organization, cf. the case of Urtica dioica (Sting Nettle) of the Urticaceae, that of 

 Mercurialis as the last elementary type of the Euphorbiaceae farthest north, and the remarkable 

 specialization of Pteris aquilina (Bracken) as the preponderant indigenous fem. 



2 Schimper (1903), loc. cit., p. 25. 



