476 BOTANY 



former instance the symbiosis is antagonistic, in the latter, recip- 

 rocal. Certain true parasites, such as Saprolegnia ferax, may also 

 live on dead matter, and are known as "Facultative parasites," while 

 others, such as some of the Rusts, can live only upon living tissues, 

 and are " Obligate " parasites. 



Certain organisms in dying (or during their growth) provide con- 

 ditions suitable for the growth of others, as in the disintegration of 

 rocks and accumulation of humus by the lower plants, preparatory 

 to the establishment of the higher ones. This is also the case among 

 certain Bacteria which appear in succession in decomposing sub- 

 stances, each preparing the way for its successor. Thus putrefac- 

 tive Bacteria are followed by Sulphur Bacteria and aerobic by 

 anaerobic forms, and vice versa. 



Among the higher plants all degrees of parasitism and sapro- 

 phytism are found. Thus the species of Mistletoe (Viscum, Phora- 

 dendron) have chlorophyll, but are to some extent parasitic, although 

 it has been questioned whether they obtain any organic food from 

 the host. Certain Scrophulariacese, e.g. Gerardia, attach themselves 

 to the roots of other plants and behave much like the Mistletoe. 

 Other forms, like the Beech-drops (Epiphegus), Orobanche, and the 

 tropical Kafflesiacese, are quite destitute of chlorophyll, and must 

 obtain all their food from the host. 



The penetration of the roots (or haustoria) of- the parasite into the 

 host is effected partly by pressure and partly by the aid of excreted 

 enzymes, or ferments. The latter are substances of great importance in 

 the economy of all plants, as they possess the power of dissolving very 

 resistant organic substances, such as cellulose, starch, and even chitin. 



Mycorhiza. Many saprophytic plants growing in a humus soil, 

 and some which are not saprophytes, have associated with their 

 roots certain Fungi, whose mycelium either lives within the cells of 

 the host (endophytic) or upon the outside of the root (epiphytic). 

 This mycorhiza, as it has been called, is supposed to supply the 

 host with certain elements derived from the decomposition of the 

 humus, in return for other substances taken from the host. Endo- 

 phytic mycorhizse occur in the gametophyte of species of Botrychium 

 and Lycopodium, as well as in the roots of their sporophytes, and in 

 those of many of the Seed-plants, notably such saprophytic forms as 

 Monotropa, Corallorhiza, etc. The mycorhiza found about the roots 

 of the cupuliferous trees is usually epiphytic, and its close associa- 

 tion with the host may be doubted. 



Assimilation of Oxygen and Hydrogen 



Oxygen and hydrogen, which, next to carbon, are present in the 

 greatest quantity in plant-tissues, are derived from water, C0 2 , or 



