42 Plant-life of the Oxford District 



bundle-distribution of the older leaves of the land in skeleton-form. In 

 such manner the wholly submerged shoot becomes free of water to any 

 depth, so long as light-penetration is sufficiently good (i. e., in clean water) ; 

 and streams and lakes fill up with subaqueous vegetation, imitating in its 

 dense ramification the vegetation of the subaerial world, and recalling in 

 such retained form-factors the older vegetation of the sea, in which such 

 structural features were originally established in response to the necessities 

 of a moving medium. Though now so essentially distinct in acquired 

 anatomy of the land-plant, the general appearance of such submerged 

 aquatics is curiously algal, in dimensions as well as in texture (cf. Potamoge- 

 ton lucens in a strong current, P. pectinatus, P. crispus, Myriophyllum and 

 Rammculus fluitans as 6 ft. trails). 



V. All aquatics so far mentioned remain attached to the bottom, and 

 are so far anchored, even when wholly submerged in the manner of benthic 

 algae, and they may grow in deep water, if this is only sufficiently clear. 1 

 Yet here, again, the mechanism of attachment by adventitious roots is the 

 essential factor : where this fails, the plants are set free to rise to the sur- 

 face in virtue of photosynthetic oxygen, to become free-floating vegetation, 

 solving the problem of optimum surface, but left free to drift at the mercy 

 of current and wind, and with a feebler absorption system than ever before. 

 Such a new type of vegetation can attain no great dominance, any more 

 than do free-floating algal forms of the sea. 2 Flotation depends entirely on 

 oxygen storage ; sinking in deep water may be fatal ; only by remaining at 

 the surface, and eliminating chances of drift (by gregarious mass-effects) can 

 any benefit be gained. Thus the minor types of plant which have adopted 

 this life, tend to imitate the broad expansions of the leaf of * floating 

 aquatics' (Lemna, Hydrocharis), while others are little advanced beyond 

 submerged forms (Elodea^ Lemna trisulca). The dilution of the food-solution, 

 the diminution of transpiration in forms floating in rather than on the 

 water, and the feeble capacity for absorption, tend to ultimate stages of 

 reduction, which render some of these forms (cf. Wolffia) the most diminu- 

 tive of Flowering Plants. 3 



VI. It is important to note that, so far, the general construction of the 

 vegetative and photosynthetic shoot-system has been alone considered ; 

 but similar conditions of failure may characterize the portion of the shoot - 

 system set apart for reproductive functions, as special inflorescence-region 

 of the plant, inflorescence-axis, and even flower, wholly independently. 

 The two regions of the plant-body are not exactly on the same footing, and 

 have to meet quite distinct problems : since the vegetative portion has to 

 bear the onus of obtaining the food-supplies, while the reproductive shoot 

 with its cycle of reproductive phases may be restricted to the more favour- 

 able period of the year. As a rule, the reproductive portion will be found 

 to be more * conservative ', to retain much more evidence of the older phase 

 of land organization, and to lag behind the advancing specialization of the 

 photosynthetic leaves. The flowering scheme of an aquatic has little refer- 

 ence to what its leafy shoots may be doing, and throughout the range of 



1 Cf. Schimper (1903), Plant Geography, p. 792, for Posidonia in the Mediterranean at 

 30-50 fathoms. 



8 Pistia and Eichornia as free-floating forms of tropical rivers become pests, as the gregarious 

 habit gives them a stationary character. Cf. Arber (1920), p. 213. 



3 These classes are by no means exhaustive, and types grade into one another even in the ontogeny 

 of a single individual ; that is to say a certain range of plasticity in the organism may obtain. 

 Batrachian Ranunculi may produce both ' broadened ' floating leaves and ' dissected ' submerged 

 laminae on the same shoot. Sagittaria with normally erected subaerial ' arrow ' leaves in rush- 

 habit, in deeper water gives arrow-forms broadening and floating on the surface, and in still deeper 

 water the attenuated ribbon-petiole (6 ft.). Free floating forms are commonly mud-plants with little 

 holdfast. 



