72 PLANT PHYSIOLOGY 



similar to the pitcher plant by digesting insects drowned in the 

 water that collects in the bases of the connate leaves. Also Zam- 

 belli (1929) says that Petunia, with the aid of the glandular hairs 

 on the aerial parts of the plant, captures insects and digests them 

 by means of proteolytic enzymes secreted by these same hairs. 

 It is a common observation that green algae such as Ulva are 

 especially common around the outlets of sewers where the water 

 is rich in organic matter. Many algae have been cultivated in the 

 dark when organic food was added to the nutrient solution, thus 

 changing over directly from autotrophic plants to saprophytes. 

 Saposchnikov (1891) showed that Dombeya leaves which were freed 

 from starch in the dark formed about 5 grams of starch per square 

 meter of leaf surface in seven days when floated upon a 20% 

 solution of cane sugar; and peanut embryos can develop for some 

 time on various sugar solutions. 



Roots are able to absorb sugar much more easily than leaves, and 

 Knudson (1916) and others have shown that corn, beans, radishes, 

 etc., can supplement their carbohydrates by absorbing sugar from 

 sugar solutions. Although absorption is favored and augmented 

 by light, it will take place in the dark, and it is quite possible to 

 raise seed plants in darkness for the entire period of their lives on 

 solutions of sugars and other nutrients. 



Breazeale (1923) also has shown that roots can absorb the car- 

 bonate ion from the soil carbonates, but he thinks there is no 

 reason to suppose that the carbon thus absorbed replaces to any 

 extent that absorbed from the air. The work of Stoklasa (1927) 

 and others cited in Chapter III may require us, however, to re- 

 adjust our conclusions on this matter. 



Relation of Photosynthesis to Plant Structure.— In concluding 

 this subject of photosynthesis, it is fitting at this place to remind 

 the reader that, while it is impossible to say that structure de- 

 termines function more than function determines structure, no- 

 where else is the interdependence of these two more clearly seen 

 than in this matter of photosynthesis. The important points in 

 their structure which separate plants from animals, as has been 

 pointed out by Ganong, are all connected with their food supply 

 and the ability of the green plant to elaborate not only its own 

 food but that for all other organisms as well : 



1. Animals must get their own food from outside. Plants can 

 manufacture it from materials which circulate by them. These 



