OTHER WAYS OF PROCURING FOOD 65 



for the great variety of climatic conditions found at various places 

 on the surface of the earth, ranging from the moist, hot tropics 

 to the cool, subarctic regions and the dry, nearly barren deserts. 

 Autotrophic Plants. — By far the great majority of the Sperm a- 

 tophytes are autotrophic, i. e., they are able to manufacture their 

 own organic food with the aid of the chlorophyll they contain. 

 A few, however, have lost this power and have become dependent 

 either entirely or partially upon food produced by other organisms; 

 such plants are said to be heterotrophic. From what has been 

 previously said, it is clear that photosynthetic power depends 

 upon the presence of chlorophyll, and only those seed plants can 

 manufacture their own food which contain this necessary pigment. 



Saprophytes.— Now it is obvious that a plant which does not 

 make its own food must get it from one of two sources, — either 

 from dead organisms or from living ones. A few of the Spermato- 

 phytes are equipped for the former habit, and live from the or- 

 ganic matter contained in decaying plant and animal matter. 

 The Indian pipe (Monotropa) is a familiar example in rich woods, 

 where it is found living on the food to be obtained in humus. From 

 the white stems, which lack chlorophyll, it has received the name 

 of "ghost plant." Since it does not make its own food there is 

 no need for leaves, which accordingly are reduced to mere scales. . 

 Such plants which obtain their organic food from dead sources 

 are called saprophytes. 



Symbiosis. — Plants which get their nourishment from living 

 organisms are known as parasites and illustrate one phase of sym- 

 biosis, which is the living together of any two organisms of two 

 different species. The word [from syn (Gr., together) and bios 

 (life)] has no reference to the nature of the relationship and 

 implies nothing whatever as to the benefits to be derived from 

 the union. There are hence two types of symbiosis. In the first in- 

 stance we shall consider, the union may be of mutual advantage, 

 in which it is impossible to say that one of the symbionts is receiv- 

 ing more from the partnership than the other. This is mutualism. 

 True mutualism among the seed plants is very rare, although 

 many cases exist between seed plants and fungi and will be dis- 

 cussed in the following chapter. 



Grafting.— In grafting, a branch or bud of one plant is united 

 to the stem of another. This union between the two stems is 

 made in such a way as to bring the greatest possible amount of the 



