294 THE BIOLOGY OF FLOWERING PLANTS 



Submerged dissected, and floating leaves may occur on the 

 same individual, as in some of the water crowfoots. The 

 very long, linear leaf is another common submerged type. 

 It, too, offers little resistance to currents. The pondweeds 

 show this form in some species which may have floating 

 leaves as well. The arrow-head, the water plantain, the 

 flowering rush have submerged linear leaves and subaerial 

 leaves of various shapes. 



Submerged flowering plants in temperate climates are 

 not found in violently rushing water ; they are confined to 

 streams flowing gently over sandy or muddy bottoms in 

 which the plants root. Those subject to most violent 

 water movement are the few peculiar flowering plants of 

 the sea, represented, on British coasts, by two species of 

 Zostera, the grass-wracks. This and other genera, such as 

 Cymodocea and Posidonia, are very common in the 

 Mediterranean. They grow on flat, muddy or sandy shores, 

 and may be left uncovered by the retreat of the tide. They 

 root in the sand, and the root system is well developed, 

 though not remarkably so. The leaves are Hnear, and, in 

 Zostera marina, may reach a yard in length. These plants 

 do not seem to be thoroughly fitted to meet the great force 

 of tide and wave in a trying environment, for they are 

 readily torn up by storms and cast on the shore in enormous 

 masses. 



In torrential streams of the tropics of America, and, to a 

 lesser extent, in Africa and Asia, there occur two remarkable 

 families, the Podostemaceae and Tristichaceas. They are 

 not found in quiet streams. The plants are attached to 

 the rocks and are firmly fixed even on smooth water-worn 

 stones. In simple cases a branched, dorsiventral root 

 system creeps over the rock face ; on the lower side 

 exogenous attachment organs (haptera) are formed, which 

 are closely applied to the inequalities of the stone, and may 

 exude a sticky adhesive material, e.g. in Tristicha. This 

 mode of attachment is analogous to that of the tendrils of 

 certain vines such as the Virginia creeper. In other cases 

 the root system develops as a thallus, e.g. in Dicraea and 



