106 SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS. 
Often the trees only serve as supports, by means of which the plants in question 
raise themselves out of darkness into light. Such food-salts as they require they 
take, not from their support, but from the earth, into which they send absorptive 
roots. As years go by, a quantity of inorganic dust collects in the forks of 
branches and in the little rents and fissures in the bark of old trees, and this dust 
gets mixed with crumbled particles of bark. The clefts, therefore, are more or 
less full of vegetable mould, and this forms an excellent foster-soil for a large 
number of plants. But it is not necessarily the case that all plants rooting in this 
mould take up organie compounds from it. Thus, one finds not infrequently in the 
angles of bifurcation of the trunks of old limes and other trees, little gooseberry 
and elder bushes, and bitter-sweet plants, which have germinated there from fruits 
brought by black-birds, thrushes, and other frugivora. These shrubs, in the forks 
of limes and poplars hardly take any organic compounds from the mould in which 
they are rooted, but confine themselves to the absorption of such mineral salts as 
they may require. 
But, with the exception of instances of that kind, the great majority of plants, 
nestling in the mould in crevices of bark, do take nutriment from this their 
substratum in the form of organic compounds. In cold regions the plants living 
in the mould of bark are for the most part mosses and liverworts. They cover 
trunks and branches of old ashes, poplars, and oaks, with a thick green mantle, and 
grow especially on the weather-side of the trees. In the tropics, on the other hand, 
the fissured bark of trees is a rallying ground not only for delicate mosses and 
moss-like Lycopodia, but also for a whole host of ferns and vivid flowering plants. 
The number of small ferns which develop and unroll their fronds from chinks in 
the bark of trees is so great that old trunks appear wrapped in a regular foliage of 
fern-fronds. Of Phanerogams, in particular, the Aroidew, Orchidacee, Bromeliacee, 
Dorstenie Begoniacee, and even Cactacew (species of the genera Cereus and 
Rhipsalis) bury their roots in the mould of bark. It is to be remarked that the 
rosettes of Bromeliacee ornament chiefly the forks of trunks, whilst Dorstenie, 
Orchidee, and the various species of Rhipsalis grow on the upper side of branches 
that ramify horizontally; whilst, lastly, Aroidee and Begoni® take root, for the 
most part, on the surfaces of huge erect trunks. 
Besides the mould collected in crevices and fissures of bark, the bark itself, that 
is, the cortical layer, dead but not yet crumbled and mouldered into dust, forms 
a nutrient substratum for a whole series of plants of most various affinity. 
Many fungi and lichens penetrate deeply the compact bark, and their hyphal 
filaments ramify between its dead cells. Other plants, instead of piercing through 
the substance of the bark, lay themselves flat upon its surface, and grow to it so 
firmly that if one tries to lift them away from the substratum, either part of the 
latter breaks off, or the adnate cell-strata are rent, but there is no separation of the 
one from the other. If a tuft of moss (eg. Orthotrichum fallax, O. tenellum, or 
O. pallens), growing on bark, or a liverwort (eg. Frullania dilatata) closely 
adherent to a similar basis, is forcibly removed, little fragments of the bark may be 
