NATURE OF PLANTS 63 



lime, potash, etc., must be added to the soil to replace the ab- 

 sorbed foods. 



The majority of our trees and 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 and are known as mycorrhiza. 

 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 in the soil and 

 transfer a portion of this food to the root and doubtless secure in 

 return certain substances from the root. This mutually helpful 

 relationship of two plants is termed commensalism. The maj- 

 ority 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 has been induced in many of these plants by cultivation. If 

 they are left to themselves in a wild condition they will soon 

 revert to an annual growth, i. e., producing seed and perishing 



