102 SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES. 
in the vegetable mould saturated by snow and rain on the ground of forests or in 
the humus covering meadows, and which percolate through into the sand or loam 
beneath. Plants whose roots ramify in this deeper layer of earth derive thence the 
organic compounds conveyed by the water, and have the additional advantage of 
being able to satisfy at the same time their requirements as regards mineral sub- 
stances. This circumstance is of importance not only to flowering-plants but also 
to many fungi, as, for instance, to all species of Phallus, they having need of a 
great deal of lime. An explanation is thus afforded of the fact, formerly difficult to 
understand, that in forests and meadows not only the upper black or brown humus 
layer, but also the underlying yellow loam, or pale sand, neither of which latter 
contains any humus, has mycelia of fungi running through it in every direction, 
and weaving their threads over little fragments of rock. Indeed, it sometimes 
happens that the lower layer of earth is more abundantly penetrated with plexuses 
of hyphe than is the upper layer, consisting of vegetable mould. The greatest 
number of saprophytes is to be found therefore at places where the humus layer is 
not too thick and loam or sand occurs at no great depth; but where decaying 
vegetable remains are piled metres high, as on moors, for example, instead of fungi 
being produced in extraordinary abundance, as one might expect, only a few occur. 
Pure peat is by no means a favourable soil for fungi, a circumstance which may be 
partly due to the antiseptic action of certain compounds developed in it. 
It follows from the foregoing observations that a sure conclusion as to the 
nature of plants rooted in a particular substratum cannot possibly be derived from 
the mere appearance of the substratum. Moreover, the conditions necessary for the 
erowth of plants requiring organic products of decay as nutriment appear to be of 
much wider occurrence than one would suppose upon a cursory observation of the 
conditions existing in fields and forests, or, if one considers exclusively instances of 
cultivated plants reared on arable land, which is manured and constantly turned 
over. The great variety of plants produced on a limited area is also now 
intelligible. From the same soil some absorb organic compounds, others mineral 
substances only; whilst others again take some organic and some mineral food- 
salts. The determining factor is not the amount of a given substance present 
in the substratum, but rather the special needs of each species, and ultimately the 
specific constitution of the protoplasm in each one of the plants which thus, side by 
side, nourish themselves in totally different ways. 
If, then, neither the appearance of the ground nor its richness in respect of 
humus affords any certain indication as to whether a particular plant lives on 
organic products of decay or not, the question may perhaps be solved by the fact 
of the plant’s containing or not containing green chlorophyll-corpuscles. We may 
take it as proved by many results of investigation, that the decomposition of the 
carbon-dioxide absorbed by a plant from the air, and the formation of the organic 
compounds of carbon, hydrogen, and oxygen known as carbohydrates (which play 
so important a part in vegetable economy), only take place in organs possessing the 
green pigment known as chlorophyll. We shall return to a discussion of these 
