222 BOTANY part i 



be placed in order cas regards their nutritive value for any particular 

 organism, this cannot be done generally ; there are many saprophytes 

 which are adapted to quite peculiar conditions and use in preference, 

 as a source of their carbon supply, compounds, which for the majority 

 of other plants have scarcely any nutritive value {e.g. formic acid, 

 oxalic acid). 



Even the saprophytes which succeed on very various compounds of carbon 

 (omnivorous saprophytes) are exclusive in their selection of a substratum. Thus 

 from ordinary tartaric acid PenicilKuvi only utilises the dextro-rotatory form, and 

 Bacillus subtilis only the laevo-rotatory form. AsiKrcjillus growing in a mixture 

 of glucose and glycerine utilises the former first (" election " of nutritive materials). 

 If the glycerine alone is given, it is completely utilised. 



The power possessed by many Fungi of utilising such organic 

 compounds as starch, cellulose, etc., which are insoluble in water, is 

 very remarkable ; these substances can only be absorbed after a process 

 of transformation and solution. The Fungi and Bacteria in question 

 excrete enzymes (cf. p. 235), which have the power of rendering the 

 substances soluble. 



Saprophytes are thus characterised by the nature of their 

 assimilatory activity ; they are unable to carry out the first 

 step in the assimilation of carbon which is eftected so easily by the 

 green plant with the heljJ of light. On the other hand there is 

 probably no difference between them and autotrophic plants in 

 the further steps of assimilation, in the construction from simple 

 organic compounds of the more or less complex compounds which 

 compose the body of the plant. 



Among phanerogamic plants also some heterotrophic forms, that at 

 first sight appear to be saprophytes, occur. This is the case for certain 

 orchids which grow in humus {Neoitia, CoraIliorhi:a, Ejnpogon) and 

 for Monotropa. The absence of chlorophyll and, except for the 

 inflorescence, the subterranean mode of life indicate the heterotrophic 

 nature of these forms. The obvious assumption that they obtain 

 their supply of carbon from the humus of the soil of woods is, however, 

 very improbable. Since all these plants harbour a fungus in their roots 

 or rhizomes, the absorption of food material is probably due to the 

 fungus. The flowering plants probably lead a parasitic life upon the 

 fungi in their roots (mycorhiza ; cf. p. 232). 



These plants thus lead us to the consideration of Parasites, 

 numerous examples of which are found in the Fungi and Bacteria ; 

 parasitic forms also occur among Algae, Cyanophyceae, and the higher 

 plants. 



That these parasites, or at least many of them, absorb nutrient 

 materials fi'om the host upon which they live is often evident 

 from the condition of the latter ; the host may be seriously injured 

 and even ultimately killed 1)y the parasite. AVhat the particular 



