102 SAPROPHYTES AND THEIR RELATION' TO DECAYIXG 150DIES. 



in the vegetable inoulil saturated by snow and rain on the gnjund of forests or in 

 the humus covering meadows, and which pei-colate through into the sand or loam 

 beneath. Plants whose roots ramify in this deeper layer of earth derive thence the 

 oi-ganic 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, thej- having need of a 

 great deal of lime. An explanation is thus afforded of the fact, fonnerlj' difficult to 

 understand, that in forests and meadows not only the upper black or In-own 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 hyphfe 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 decaj-ing 

 vegetable remains are piled metres high, as on moors, for example, instead of fungi 

 being produced in extraordinary abundance, as one might expect, onlj^ a few occur. 

 Pure peat is by no means a favourable soil for fungi, a circumstance which ma}' be 

 partly due to the antiseptic action of certain compounds developed in it. 



It follows from the foretjoingf observations that a sure conclusion as to the 

 nature of plants rooted in a particular substratum cannot possibly be derived from 

 the mere appeai-aiice of the substratum. Moi'eover, the conditions necessaiy for the 

 growth 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 diflerent 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 bj- the fact 

 of the plant's containing or not containing green chlorophj^ll-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 carbohj^drates (which plaj' 

 so important a part in vegetable economy), onl}' take place in organs possessing the 

 green pigment known a.s chlorophyll. We sliall return to a discussion of these 



