NATURE OF PLANTS 59 



is of prime importance is successful farming. Each kind of plant 

 takes from the soil certain elements and for this reason the alter- 

 nating or succeeding crop should be of such a kind as to require 

 other elements. As these elements are removed from the soil 

 suitable fertilizers containing nitrogen, phosphorus, lime, potash, 

 etc., must be added to the soil to replace the absorbed foods. 



The majority of our trees ajid many of our shrubs are depen- 

 dent upon certain fungi at least for such foods as are derived from 

 humus. These fungi consist of delicate, cobwebby threads such 

 as are seen on mouldy bread. These threads spread through the 

 soil and either enter the outer cells of the root or simply form a 

 mantle about the root. The fungi live upon decaying animal and 

 plant life and transfer a portion of this food to the root and 

 doubtless secures in return certain substances from the root. 

 This mutually helpful relationship of two plants is termed com- 

 mensalism. The majority of our heaths, evergreens, poplars, 

 willows, beeches, and oaks have become dependent upon these 

 fungi and do not thrive in soils where the fungi are not found. 

 Perhaps this explains why it is so difficult to transplant certain 

 shrubs and trees. The fungi are easily injured and do not become 

 established in the new soil soon enough to keep the plant alive. 

 Some plants have become so perfectly adapted to these fungi as to 

 receive all necessary foods from them. Accordingly, their green 

 leaves and roots have largely disappeared since they are no longer 

 of service. Examples of this are seen in the white Indian pipe 

 and pine-sap and in the coral root orchid. 



28. Roots as Store-houses for Foods. In all the cases hereto- 

 fore considered the root has functioned in one way or another as 

 an absorbing organ. It may also serve in other capacities, one 

 of the more important of which is as a storage organ. Such roots 

 become fleshy and filled with foods and are of great economic 

 importance, furnishing a variety of nutritious vegetables, as the 

 sweet potato, beet, turnip, etc. Many of these valuable plants 

 are biennials. During the first season the plant develops only 

 leaves and stores up food in its fleshy roots which is utilized in 

 the following season in the production of flowers and seeds, after 

 which the plant perishes. It is noteworthy that the biennial habit 



