USES OF THE PHOTOSYNTHETIC FOOD 



273 



to catching insects. Plants with such leaves are often called 

 "carnivorous plants " or "insectivorous plants." The "Pitcher 

 Plants " are so named because the leaves form tubes or urns of 

 various forms, which contain water, and to these pitchers insects 

 are attracted and then drowned. (Fig. 245.) The plants known 

 as "Sundews" have their leaves spread on the ground and 

 clothed with secreting hairs. 

 {Fig. 2Jfi.) These secre- 

 tions not only entangle in- 

 sects but digest them. In 

 the "Venus Flytrap," por- 

 tions of the leaves work like 

 steel traps and hold the in- 

 sects fast until digested. 

 {Fig. 247.) 



Transformations of the 

 Photosynthetic food 



After the photosynthetic 

 food has been formed in the 

 green parts of plants and dis- 

 tributed to the various re- 

 gions of growth, next follows 

 the transformation of the 

 photosynthetic food into the Fig. 247. —Venus Flytrap, showing 

 numerous other kinds of sub- the leaves which open and close in catch- 

 stances. Carbon, hydrogen, "^2'°^^'=*^ 



and oxygen," the elements of which the photosynthetic sugar is 

 composed, as its formula CgHiaOB shows, constitute 90 percent 

 or more of the dry weight of most plants. Some of the hydro- 

 gen and oxygen are present in the form of water, a little of which 

 remains in plants despite the long drying in ovens to reduce them 

 to their dry weight, but the carbon and much the greate.- part of 

 the hydrogen and oxygen in the dry weight of plants have come 

 from the photosynthetic sugar. Usually the mineral elements, 

 which represent the constituents taken from the soil, constitute 

 no more than 10 percent and often less than 5 percent of the dry 

 weight of plants. It is obvious that plants are built chiefly out 

 of the constituents furnished by the photosynthetic sugar. It 

 furnishes all of the elements for many of the plant substances 



