364 THE FOOD OF PLANTS 



the external world, the rest being supplied by the imperfectly developed 

 chlorophyll-apparatus ; such may be termed ' mixotrophic ' plants. Even 

 a typical autotrophic 1 plant may be artificially nourished to a certain 

 extent, although it can obtain all the organic material it requires by 

 photo- or chemo-synthetic assimilation. Similarly many plants live for a 

 time parasitically, although when fully developed they may be capable of 

 decomposing carbon dioxide, as is the case, for example, in seedlings which 

 feed at first upon the food-materials of the seed, and also in the under- 

 ground rhizomes of certain orchids which live saprophytically for a time. 



Neglecting the innumerable adaptations which heterotrophic and 

 mixotrophic plants have developed in order to obtain a sufficiency of 

 organic food from without, it is only possible to draw a broad distinction 

 between saprophytes and symbionts. Saprophytes obtain their nutriment 

 from dead organic material, from the bodies of animals or plants, or from 

 natural or artificial nutritive solutions. In those cases in which the food 

 is obtained directly from living organisms we have examples of conjunctive 

 symbiosis no matter whether the symbiont is epiphytic or endophytic 2 . 

 Lichens form an example of reciprocal symbiosis, in which the union 

 confers advantages upon both partners ; when on the other hand the host- 

 plant is simply preyed upon by a parasite the symbiosis is antagonistic. 



These distinctions cannot, however, be rigidly adhered to, for many 

 fungi may grow indifferently cither as parasites or as saprophytes, or 

 may regularly change from the one mode of existence to the other, while 

 others have been successfully cultivated as saprophytes although under 

 normal conditions they are obligate parasites a . Further it is often doubtful 

 whether a supposed example of reciprocal symbiosis is not really a case of 

 parasitism in which the host-plant is not injured to any great extent, but 

 perhaps receives partial compensatory advantages, and even the most exact 

 mutualism may be disturbed under certain circumstances, as for example 

 when a scarcity of food material prevails, or when the conditions become 

 abnormal in some way or other. All stages of transition between pure 

 autotrophism and heterotrophism are exhibited among obligate or faculta- 

 tive mixotrophic plants. (Cf. Sect 50.) 



The balance of nature is such that various degrees of dependency exist 

 between all forms of life, for every organic substance is directly or indirectly 

 derived from the photosynthetic assimilation of carbon dioxide, and hence it 

 is ultimately from this source that both saprophytic and parasitic plants and 

 animals derive their food, while even those organisms which utilize the 

 ultimate products of decomposition also obtain their energy indirectly 



1 Frank (Lehrb. d. Bot., 1892, p. 548) has used these terms in a somewhat different sense. 

 * De Bary, Erscheinungen d. Symbiose, 1879, pp. 6, 21, &c. Cf. also Reinke, Jahrb. f. wiss. 

 Bot., 1894, Bd. xxvi, p. 526. 



3 Cf. de Bary, Fungi, Mycetozoa and Bacteria, 1884, p. 381 ; Zopf, Die Pilze, 1890, p. 228. 



