854 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
Considering hairiness alone, we have seen that its distribution on the 
plant is correlated in the most minute way with differences of external 
conditions ; that those parts of the plant which would, if the conditions 
were severe, tend to lose water most readily, are those which are furnished 
with this hairy covering ; and further, that the hairy type is more success- 
ful than the less hairy var. denudata . It may therefore be concluded that 
hairiness in this species is a positive advantage to the plant, and in all 
probability actually necessary whenever transpiration conditions are unusually 
severe, e. g. in the event of strong drying winds or prolonged insolation. The 
same may be said of the other xerophytic devices enumerated above. In 
other words, Spiraea Ulmaria is structurally in complete harmony with its 
environment. 
But so far only the aerial organs have been considered, and it may well 
be asked, why does a plant rooted in damp soil require these regulatory 
devices at all ? And further, even if the necessity be proved for Spiraea , 
how far does this particular instance afford a clue to the problem of xero- 
morphy in marsh and bog plants in general ? An attempt will be made in 
the next section to give a provisional answer to these questions. 
§ 14. The Present Position of our Knowledge of ‘ Swamp 
Xerophytes’, and a Tentative Hypothesis. 
Xeromorphy in plants growing on wet, peaty soils occurs all the world 
over . 1 A full historical account of the various theories put forward to 
account for the existence of these ‘ swamp xerophytes ’ need not be given 
here ; but the positions taken up by certain authors will be briefly 
discussed. 
Some writers maintain that the xeromorphic structures in question are, 
and others that they are not, present-day necessities to the plants which 
possess them. 
The latter point of view may be dealt with first. Schwendener , 2 
Clements , 3 and others believe that these structures are primitive characters 
which have persisted in certain stable species (especially Monocotyledones), 
in spite of a fundamental change of habitat conditions. Perhaps the most 
important evidence for this view is the fact that hydrophytic species 
frequently grow side by side in nature with those possessing xerophytic 
characters. Even Warming, who believes that the xeromorphy is adap- 
become so because it is sufficiently adapted to its whole environment, not only at any one time or to 
any average of conditions, but to the most extreme adverse conditions which have occurred during the 
thousands or millions of years of its existence as a species.’ 
1 Warming (’ 09 ), p. 194. 2 Schwendener (’ 89 ), p. 73. 
3 Clements (’ 05 ), p. 126. 
