140 TYPES OF LEAF IN WATER PLANTS [CH. 



he shows that a cylindrical leaf, in order to have as high a ratio of 

 surface to volume as, for example, the broad, flat leaf of Pota- 

 mogeton -perfoliatus, must be only 120^ in diameter, whereas the 

 diameter of the segments of Myriophyllum spicatum leaves varies 

 from 22o/t to 38o/z, and of Ceratophyllum demersum, from 6oo/x 

 to 75o/x, while even the ultimate divisions of the leaves of Ra- 

 nunculus trichophyllus reach 1 90/^1. He admits that, apart from the 

 actual ratio of surface to volume, the dissected leaf may possibly 

 have an advantage over the corresponding flat leaf, in tapping a 

 greater volume of the medium 1 ; he thinks, however, that though 

 this factor would be of importance in absolutely still water, its 

 significance is much reduced if, as is nearly always the case, 

 movement has to be taken into account. It may be added that the 

 dissected leaf possibly interferes less with its neighbours' light 

 than the undivided type of submerged leaf. From this enquiry 

 and from a general study of submerged leaves, it may perhaps 

 be concluded that both the dissected and flat types of leaf are 

 organs of tolerably equal efficiency for subaqueous gaseous 

 exchange, though the dissected leaf has the advantage of 

 offering less resistance to currents. Which type of leaf a sub- 

 merged plant shall produce is probably ultimately decided by 

 the general leaf morphology of its terrestrial ancestors, rather 

 than by environmental causes, much as coast scenery is often 

 determined by the forms of the pre-existing land surfaces, 

 rather than by the direct action of the ocean itself. 

 _'. Among the undissected types of submerged leaf, the ribbon 

 leaf is conspicuous (Fig. 90) ; it is probably better adapted to 

 resist tearing than, for instance, the large, U/va-like submerged 

 laminae of the Waterlilies. Ribbon leaves are found among 

 many of the marine Angiosperms, such as Zostera, which are 

 subjected to the wash of waves and tide. Leaves of this type 

 sometimes grow to a notable length ; those of Sagittaria sagitti- 

 folia, as we have shown in Chapter n, may be more than two 

 yards long, while those o>i V allisneria spiralis are said to be often 

 a yard or more in length, though hardly a quarter of an inch wide. 

 iSchenck, H. (1885). 



