THE NUTEITION OF THE EOOT 47 



purely mineral soil, or if the humus has been sterilised by 

 heating, and the fungus is therefore killed. According to 

 A. B. Frank, upon whose extensive investigations these re- 

 marks are based, mycorhiza is also present in heaths, in most 

 orchids, in many Liliacese and Smilacese, and in a large 

 number of OompositEe, Labiatae, Primulacese, Umbelliferae, 

 Rosacese, Leguminosse, and Ranunculacese. 



The fungi which form the mycorhiza belong to those 

 kinds which usually occur in the humus of woods and fields. 

 They do not seem to act in any way injuriously on the root, 

 but, on the contrary, stimulate it to renewed growth, and no 

 doubt the root in its turn stimulates the growth of the fungus, 

 nourishing it with its own sap. We have here, therefore, an 

 example of the living together or partnership of two organisms 

 {symMosis), each of which benefits the other. The fungus, too, 

 is not always restricted to the outside of the epidermis, or to 

 making its way between the epidermal cells (ectotrophic mycor- 

 hiza), but it may live exclusively within the living cells of the 

 root (endotrophic mycorhiza). In this latter case the roots 

 need not necessarily present the short, thick, and much- 

 branched condition, but may be long and very slender, and 

 devoid of cortical tissues. The fungus fills the epidermal 

 cells, which then lie immediately outside the central vascular 

 cylinder. In those roots which possess cortical parenchyma, 

 the endotrophic mycorhiza occupies a ring-like zone of cells, 

 from which fungal threads run towards the outer surface of 

 the root. 



While the symbiotic association of these green plants with 

 a fungus is only formed when they grow in humus, and is 

 more largely developed as the soil becomes richer in humus, it 

 occurs always in plants devoid of chlorophyll, and there the 

 mycorhiza is either ectotrophic or endotrophic. In these 

 plants, therefore, the mycorhiza appears as a necessary means 

 of absorption, while in the chlorophyll-containing plants it is 

 only occasional and additional means of nutrition. Thus we 

 find in the absorption and assimilation of decomposed organic 

 matter a parallel to the conditions obtaining in parasitism, i.e., 

 in the absorption of living organic matter. We have obligatory 

 humus absorbers (Monotropa), to which plants humus is indis- 



