Subordinate and Herbaceous Flora 41 



III. Where in turn such mechanism of orientation and erection in the 

 petioles wholly fails, the leaves droop, to float passively on the surface of 

 the water. With such relief from all necessity of mechanical support and 

 also from the possibility of desiccation, new possibilities open up for what 

 may be termed the anchored floating leaf. In fact so many problems are 

 solved by the adoption of passively floating leaves, that their anatomical 

 organization is modified to meet the new conditions; e.g., restriction of 

 stomata to the upper surface is readily effected when they were originally 

 on both sides. Such floating leaves become characteristic of a wide range of 

 aquatics, from facultative grasses to more particularly highly specialized 

 types of Dicotyledons. To such leaves increase of surface is essential, as 

 they lie in the horizontal plane only, and the plant becomes a-dimensional 

 so far as its photosynthetic area is concerned ; while the available space 

 becomes more restricted as rhizomes are condensed and the plants are gre- 

 garious. The leaves spread out to enlarged sub-orbicular form, with leaf- 

 mosaic on the water-surface, soon occupying all the room available ; but 

 capable of indefinite extension in open water so long as it is of no great 

 depth. The mechanism of petiolar extension gives the regulation of 

 the lamina to the depth of the water and its position in the leaf-mosaic, and 

 this becomes a limiting factor : e.g., Nymphaea alba may grow in ao ft. of 

 water. Such a depth of water is beyond the capacity of any tree-type, and 

 beyond the preceding aquatic habit ; so that the floating leaf opens up 

 a new field of operation to plant-forms, in which they are beyond the com- 

 petition of the older types of the land. In thus finding a ' new station ' in 

 the water, plants are not necessarily c driven off the land ' ; the latter 

 expression remains metaphorical as a poetic exaggeration of the facts, 

 merely because we remain on the land ourselves. 1 It is difficult now to 

 visualize what such a fine plant as the Victoria regia of the Amazon may 

 have been before it left the land. 



IV. It may be noted that such petiolar mechanism has again its 



(limitations ; it cannot be extended indefinitely, and it does not meet the case 

 of casual alterations in the water-level, or seasonal periodicity of the supply. 

 That is to say, its failure may be expected ; e.g., at the beginning of the 

 growing period, or at the end of the season, many leaf-laminae may fail to 

 reach the surface at all. In such case a new type of submerged leaf may 

 come into action ; and where this proves efficient for photosynthesis a new 

 set of structural alterations may be called into operation. When so appar- 

 ently withdrawn beneath the surface, the photosynthetic area is no longer 

 2-dimensional in the horizontal plane, nor does it require to be vertically 

 elongated as in the rush-type. The necessity for the orbicular leaf vanishes, 

 though a linear form may still be useful where it lies in the flow of the 

 current. The plant-body again becomes 3-dimensional, taking all the light 

 it can receive ; the more so as effective light diminishes rapidly in intensity 

 beneath the surface of the water. Stomata are no longer required ; normal 

 transpiration is no longer effective ; the plant-form tends to regain an algal- 

 habit. Leaf-laminae reduce to thin crumpled expansions, or are dissected 

 to fine filamentous segments which space themselves in the medium, with 

 no special mechanical difficulties, admitting any light they do not use to the 

 members beneath them, and retaining in their reduction the pattern of the 



1 Arber (1920), Water Plants, p. 324. Emigrants leaving this country for the colonies may be 

 said to be similarly ' driven out ' by financial, social, political, or theological pressure ; but they 

 leave of their own free will ; they are by no means the leavings of the population ; but on the 

 contrary, are distinguished by their greater enterprise in the search for new stations. The criterion 

 of success is adaptability to new conditions ; the non-adaptable are the real failures ; and by 

 adaptability is now understood a capacity for freely mutating under the stimulus of a new environ- 

 ment. The rest is lelt to natural selection. 



