256 BOTANY PART i 



excrete enzymes (cf. p. 264), 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 effected so easily by the 

 green plant with the help 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 (Neottia, Coralliorrhisa, Epipogon) 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 (mycorrhiza ; cf. p. 261). 



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 from 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 by the parasite. What the particular 

 substances are that the parasite absorbs and requires for successful 

 growth is, however, difficult to determine. Since frequently only 

 organisms of a definite natural group (family, genus, species) are 

 attacked by one species of parasite, it may be assumed that the latter 

 makes quite specific demands as to the quality or quantity of its 

 nutriment. This assumption is supported by the fact that we are 

 unable to cultivate most parasites apart from their hosts. 



B. ASSIMILATION OF NITROGEN 



Since a green plant obtains its carbon from carbon dioxide, which 

 is only present in a very small proportion in the air, it might be 

 assumed that the enormous supply of nitrogen in the air would form 

 the primary and the best source of this element of plant food. 

 Every water culture, however, shows clearly that atmospheric nitrogen 

 cannot be utilised by the typical green plant. If combined nitrogen 

 is omitted from the nutrient solution the plant will not grow. 



