Neiv Cases of Epiyhytism among AUjce. 211 



unaffected places. These appearances become more evident 

 when the liost is densely covered by a rhizoidai plexus in 

 some given area ; and that they serve to modify not only 

 the conditions of internal tension within the host cells, but 

 also the strength and direction of the molecular diffusion 

 currents existing between adjoining cells, thereby being 

 the means of ultimately establishing altered conditions 

 throughout the entire plant, is obvious. 



3. Modifications of the host plants as a whole are also 

 sometimes brought about in virtue of the weight of the 

 epiphytes, especially in cases where the former are largely 

 infested by the latter. Thus in general a somewhat 

 greater rigidity, as well as a somewhat more stunted growth 

 of the thalli than would normally be found, are induced. 

 These results seem to be most easily explained by the 

 increased strains to which the host plants are subjected, in 

 virtue of the action of the wind or waves against the 

 increased surface afforded by the epiphytes. A necessity 

 for more efficient root-hold on the part of the host thus 

 arises, in the first instance, in order that it may secure at 

 once permanence of place and prolongation of life. The 

 rhizoids accordingly become somewhat more strongly 

 developed by an increased flow of food molecules to them, 

 this in turn reacting on the total supply of such molecules 

 that are available for the normal growth of the organism. 

 Similar conditions obtain in connection with the other 

 regions of the host, and a general diminution in the vegeta- 

 tive development ensues. The size of the thallus being 

 thus changed, it is naturally to be expected that its capacity 

 for fructification would also be diminished ; and although 

 little doubt can exist that this will ultimately be shown, 

 the effects of epiphytism have not yet been traced so far. 

 It is, moreover, of interest to note in this connection that 

 the dynamical effects^ of the epiphyte-bearing plants on the 

 shales or other substrata to which they may be affixed are 

 increased, inasmuch as the buoyant power is augmented ; 

 while contrariwise, the intensity of wave impact, and 

 consequently the erosive power of the sea, is diminished, 

 owing to tlie greater resistance offered by the associated 



^ See also "Note on Ectoearpus," Trans. Roy. Soc. Edin., vol. xxxii. pt. 3 

 1884-85, and Tram. But. Soc. Edin., vol. xiv. pt. 2. 



