NUTRITION 



38i 



Among the many thousand species of heterotrophic plants, the bac- 

 teria and fungi hold the dominant place. A few seed plants lack 

 chlorophyll entirely, such as the Indian pipe (Monotropa), beech drops 

 (Epifagus virginiana), dodder (CusaUa), etc.; and some have only par- 

 tially lost it, or with a good supply nevertheless have the nutritive habits 

 of the non-green plants. 



The families in which such dependent species are prominent are the Loranthaceae, 

 Rafflesiaceae, Scrophulariaceae, Orobanchaceae, and Balanophoraceae. 



If a plant cannot make carbohydrates, it must of necessity get food 

 directly or indirectly from some plant that can. The direct way of 

 doing this is to live on or in a live green plant. The indirect way differs 

 only in that the food secured is more remote from the original food 

 maker. Thus, a plant may live upon or in some animal or some non- 

 green plant, or upon the dead bodies of these, more or less decayed and 

 disintegrated. Indeed, decay and disintegration are only the obvious 

 evidence that plants (chiefly the minute bacteria and fungi) are living 

 upon such a dead body. And not infrequently 

 death itself is simply the result of the vigorous 

 development of such creatures on or in the 

 body of a once healthy organism. 



Parasitism. — An association between two 

 live organisms is known as symbiosis. When 

 one obtains its food from the other, the rela- 

 tion is called parasitism, and the two are known 

 respectively as parasite and host. As a rule 

 the food maker is called the host, and the other 

 the parasite; if neither or both be food makers, 

 the larger is distinguished as the host. Thus, 

 fungi are parasitic on leaves or twigs or in 

 the wood of trees, or on animals; " beech- 

 drops " (Epifagus virginiana, a small flower- 

 ing plant) is parasitic on the roots of the beech 

 tree; mistletoe is parasitic on elms, etc. This 

 relation requires the closest contact between 

 the cells of parasite and host, and the parasite 

 even penetrates the cells of the host in many 

 cases. The smaller parasites, such as fungi, 

 may grow bodily through cells, doubtless dis- 



FiG. 651. — An epidermal 

 cellof agrass(Poa) penetrated 

 by a branched haustorium (h) 

 of a fungus (Erysiphe gra mi- 

 nis); the mycelial hypha to 

 which the slender penetrating 

 tube (a) is attached is not 

 shown. — After Smith. 



