GENERAL 365 



from the sun. Each organism may therefore during its life, and also 

 after its death, provide food for "other organisms, or may create the special 

 conditions necessary for the development of certain forms. Thus certain 

 putrefactive bacteria form sulphuretted hydrogen, and render the de- 

 velopment of sulphur-bacteria possible, and other organisms by absorb- 

 ing oxygen provide appropriate conditions for the growth of anaerobic 

 bacteria. A series of forms may follow one another in a putrescent fluid, 

 each providing a suitable medium for its successor, and hence the ultimate 

 result is by no means entirely dependent upon the original character of 

 the nutrient fluid. 



Typical conjunctive symbiosis is therefore simply a case in which 

 a direct interchange of food-material occurs between two organisms. Two 

 or more organisms living together, but without any direct or fixed union 

 occurring between them, may provide the conditions necessary for their 

 conjoint existence 1 . This may be termed disjunctive symbiosis, and the 

 inter-relations existing between flowers and insects and between ants and 

 plants are examples of it. 



The different cells and organs of the same plant live together in 

 a form of conjunctive symbiosis, and every non-chlorophyllous cell is fed 

 like a heterotrophic plant by organic material obtained from assimilating 

 cells. By grafting a leafy shoot of one plant upon the root-stock of another 

 a permanent reciprocal symbiosis of similar character may be produced in 

 certain cases 2 . In many parasites union takes place between the corre- 

 sponding tissues of the host and the parasite, as is the case in members of 

 the Orobancheae, and in many species of Rhinanthus and Cuscuta. Active 

 absorption is, however, possible in the absence of any such direct fusion, 

 for a grass seedling absorbs food from the endosperm which is merely 

 apposed to it, and the embryo of a fern as well as the sporophyte of a moss 

 is nourished in the same manner. 



Heterotrophic plants vary much as regards the means by which they 

 obtain nutriment. Many forms may develop under widely different con- 

 ditions, owing to the marked accommodatory powers such plants frequently 

 possess, whereas others have a very limited range or a highly specialized 

 habitat, as is the case in the sulphur-, nitro-, and anaerobic bacteria. Many 

 saprophytic fungi and certain parasites may be grown upon artificial 

 nutrient solutions or solid media, and even when this is not found possible 

 it may simply be because the experimental conditions are not properly 

 adjusted to the plant's requirements. 



The absence of any one of the essential conditions will render develop- 



1 Various examples are given here, and in Chaps, viii and ix. Conjoint infections are also 

 examples of the same phenomenon (cf. Fliigge, Mikroorganisinen, 3. Aufl., 1896, Bd. I, p. 309). 



2 Vochting, Transplantation, 1892, p. no ; Daniel, Rev. ge"n. d. Bot., 1894, T. vi ( p. I. 



