WATER-PLANTS 
158 
phenomenon is correlated largely with contrasts in the 
amount of transpiration going on. 
Only the recent wood is employed in the carriage of 
water up to the leaves; this, with the watery wood of the 
year or two preceding it, is termed the sap-wood or 
alburnum in contradistinction to the older heart-wood or 
duramen . The outside of the stem is covered by the 
bark , a corky layer formed by a special formative tissue 
or phellogen ; between it and the wood lie the phloem and 
cambium. 
Water-Plants 1 , or hydrophytes , are a very well-marked 
form of vegetation, contrasting with xero- and meso-phytes. 
Close similarity in the conditions of life seems to have 
determined, amongst plants belonging to various different 
families, a remarkable resemblance in general habit and 
structure. In few cases can we so satisfactorily determine 
which of the peculiarities with which we have to deal are 
really adaptations to the mode of life. 
Primitive water-plants, i.e. those which have no land- 
forms among their ancestry, probably do not occur in the 
higher groups of plants with which this book deals; probably 
these are almost undoubtedly descended from land-forms, as 
is indicated by their possession of flowers adapted (in most 
cases) for pollination by wind or insects, and by many facts 
in their morphology and anatomy. They perhaps began 
as marsh plants and were gradually driven into purely 
aquatic existence by the necessities of the struggle for 
existence. The date at which the ancestral forms became 
aquatic may have been comparatively recent or very far 
back In the latter case we find entire natural orders 
composed of water-plants, as Ceratophyllaceae, Naiadaceae, 
Podostemaceae. In the former, we have instances like 
Hottonia in Primulaceae, of one or two aquatic genera in 
a family, or the water Ranunculi, where it is one or two 
species in a genus. The modification of structure to meet 
the changed conditions of life seems also to have progressed, 
1 Schenk, Die Wassergewdchse , Bonn, 1886 ; Anatomie der sub- 
mersen Gewachse{ Bibl. Bot.); Goebel, Pflanzenbiolo^ische Schilderungen ; 
Schimper, Pflanzengeographie ; Warming, Oekologische Pflanzengeo- 
graphie ; Willis, Morphology and Ecology of the Podostemaceae , Ann. 
Perac!. I., 1902, p. 267. 
