140 



CONSTRUCTION OF PLANT'S FOOD 



Food-building Apparatus. It is the special business of 

 leaves to make the food, and there the palisade and spongy 

 parenchyma are the tissues directly concerned in the work (Fig. 

 69), since the new food makes its appearance first in them. 

 Each individual cellof these tissues is therefore a food-building 

 unit, and the factors concerned in food synthesis can be fol- 

 lowed out in seeing how one of these cells does its work, namely, 



how it obtains the raw materials and the sun's 

 energy, how the arrested energy of the sun is 

 utilized, how the finished product is disposed 

 of, and how the apparatus of the cell is kept 

 in working order. 



The Chloroplasts. The chloroplasts (Fig. 

 70) are plastids that contain the green pig- 

 ment chlorophyll (see page 8). They are 

 the organs of the cell that are directly con- 

 cerned in food-construction from carbon 

 dioxide and water. This is known from the 

 fact that this process does not go on where 

 chloroplasts are absent; and where starch 

 is formed, as is nearly always the case, it 

 invariably is found .within the body of the chloroplast. 



The chloroplasts and the green chlorophyll are distinct things, 

 for alcohol will extract the chlorophyll and leave the chloro- 

 plasts as before but devoid of color. (See about origin of chloro- 

 plasts on page 9.) The chlorophyll is a pigment which the chloro- 

 plasts manufacture with the aid of the sunlight. The signifi- 

 cance of the chlorophyll is that it arrests a part of the energy 

 from the sun and transforms it in such a way that the chloro- 

 plasts can use it in food synthesis. We cannot state, and do 

 not at all know, the details of this process. Clearly the chloro- 

 plast, a living body, does the work, but to do this it needs to be 

 energized by the sun, and this is apparently what the chlorophyll 

 is instrumental in bringing about. Because the sunlight fur- 

 nishes the energy for food construction the process is called 

 photosynthesis. 



FIG. 70. Diagram- 

 matic representation of 

 a single palisade cell, 

 with chloroplasts lining 

 the walls. 



