668 I'KOCUMBENJ' AND FLOATING STEMS. 



finely-divided, submerged leaves, in addition to orbicular floating ones as in the 

 water-lilies, &c. Such plants are known as heterophyllous {pUintcje heterophyllcc). 

 Examples are furnished by sevei-al potamogetons (Potamogeton heterophyllus, 

 rufescens, spathidatus), some water-crowfoots {Ranunculus aquatilis, Baudotii, 

 hololeucus), the Cabomba (Cahomba aquatica) and the Water-chestnut (Trapa). In 

 the sixth group the plants are firmly rooted in the mud like those of the foi-mer 

 group, but the shoots rising from them bear only submerged, thin and limp leaves. 

 These plants in descriptive botany are called " submerged " {plantet submersw). 

 Their leaves — -arising from the much-branched filamentous stems — exhibit an 

 endless variety of form. They ai-e sometimes decussate, sometimes spirally 

 arranged, often broad and embracing the stem, and then again fall into the 

 ojjposite extreme, and form long very narrow ribbons or threads. Frequently they 

 are reduced to mere bristles; in other cases they are entire and undivided; again, in 

 other instances, they have finely indented and sinuous margins {cf. fig. 136, p. 551). 

 All these various forms of leaf are connected with the peculiarities of the habitat, 

 with the attacks of animals to which they are liable, with the conditions of 

 illumination at diflerent depths of water, but chiefly with the direction of the 

 foliage-stem. The long thin stems can only maintain a vertical position in still 

 water, and only in the calm inlets of lakes and in the deep pools where an active 

 movement of the water is impossible are to be found species whose submerged 

 leaves, arranged at definite intervals, exhibit a circular form. In running water, 

 especially in quickly-flowing streams, the leaves are always long drawn out, 

 ribbon-shaped, filamentous, or divided into thread-like lobes. They adapt them- 

 selves exactly to the current, and follow it in all its movements uninjured. The.se 

 leaves of running water are always fairly tough; their cell-walls are corre- 

 spondingly thickened; the stems from which they arise are protected against 

 rupture by bundles of bast deposited in the cortex, and are strengthened against 

 strains by various other contrivances to be presently described. 



While the foliage stems of the water and marsh plants hitherto described are 

 anchoi'ed fast by roots to the muddy bottoms of lakes, pools, and streams, those of 

 the Aldrovandia, figured on p. 151, and also of the bladder- worts described and 

 figured on pp. 120, 121, float in the water without a trace of root formation. Since 

 the leaves require light, it is clear that they will take up their position near the 

 surface. At any rate at the time wdien they are actively engaged in the manufac- 

 ture of organic matter under the influence of light, they are obliged to seek such 

 illuminated places. The bud-like tips of the shoots can, of course, in many species 

 sink to the bottom for the winter rest, but at the commencement of the favourable 

 season next spring, they again ascend and produce their flowering axes above the 

 surface of the water. A horizontal, or obliquely ascending position is the most 

 advantageous for the stems of these floating plants as regards the illumination of 

 their leaves, and, as a matter of fact, this direction is observed in them. Running 

 water Avould form a bad environment for such rootless, freely oscillating plants; 

 they ai'e found exclusi\^ely in the calm inlets of ponds and lakes and in pools and 



