GROWTH AND NUTRITION 21 



glycerine and fatty acids (by a group of enzymes called 

 lipases or steapsins) and these, in turn, finally converted 

 into carbohydrates (Vines, p. 173). The proteins are 

 converted back to the amide forms by proteolytic en- 

 zymes similar to the pepsin and trypsin of the digestive 

 organs of animals. 



20. Photosynthesis, or starch formation. On the ex- 

 haustion of the reserve materials of the endosperm, new 

 food must be manufactured for the growing plant from 

 similar constituents of the air and soil. The first and 

 principal foods produced are carbohydrates, chiefly starch, 

 which are formed only in green portions of the plant, 

 especially in the leaves. They are made in the chloro- 

 plasts, which bodies are abundant in the palisade cells of 

 the leaf and numerous in the spongy parenchyma. The 

 process is effected by the chlorophyll in the chloroplasts, 1 

 employing the energy of the sunlight. In the apparently 

 intricate process of formation of carbohydrates, water 

 and carbon dioxide of the air are primarily concerned, 

 the latter obtained through the stomata of the leaf and 

 the intercellular spaces. There is much more starch in 

 the leaf cells in the evening than in the morning, because 

 it is formed only in the daylight, and during darkness is 

 translocated in other portions of the plant. Moisture 

 and heat are essential in photosynthesis. Water not 

 only takes direct part in starch formation, but its presence 

 keeps the stomata open and the tissue turgid, thus favor- 

 ing photosynthetic action. The extreme range of tem- 



1 Plastids are relatively small distinct bodies, originating in 

 the protoplast (the original and simplest form of the cell) which 

 perform special functions, and are known as leucoplasts (from 

 which the others are derived), and chromoplasts (Stevens, 

 1907, p. 9). 



