PLANT PHYSIOLOGY 129 



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possesses no distinct circulatory organs, and it is believed that 

 tnese physical forces are sufficient to account for the lifting of 

 water from the soil to the leaves and buds. 



Transfer of Substances Downward. It is evident that there 

 must be a transfer of material downward as well as an ascent 

 of sap. As we shall presently notice, plants are engaged in 

 making starch in their leaves, and this starch is certainly carried 

 to all parts of the plant, since it may be stored in the under- 

 ground parts. The starch in a potato, for example, is made in 

 the leaves and hence it must be carried downward. The method 

 by which the material is carried from the leaves downward is 

 even less understood than the ascent of sap, although osmosis 

 is undoubtedly one of the factors. It is known, however, that 

 the starch is first changed to sugar and then dissolved in the 

 liquids of the plant. It is also known that these materials then 

 descend, not in the same cells in which sap is ascending, but 

 in the large sieve cells of the bark (see Fig. 46), which are the 

 cells chiefly concerned in the downward current. Since the bark 

 is needed for this downward passage of food, we see another 

 reason why the cutting of the bark away from the tree for a 

 short distance, girdling, will in time kill the plant, since the food 

 materials made in the leaves cannot then be carried to the roots 

 and they will die for lack of nourishment. 



PHOTOSYNTHESIS OR STARCH MANUFACTURE 



By the process just described, the water, with the dissolved 

 minerals, is brought to the chlorophyll cells in the leaves. These 

 same cells are also in direct contact with carbon dioxid which 

 is in the air and is brought into the leaf through the stomata. 

 The chlorophyll-containing cells have the wonderful power of 

 causing the carbon dioxid obtained from the air, and the water 

 obtained from the soil, to combine with each other chemically 

 to form a new product. The transformation is represented by 

 the following equation : 



12CO 2 -M2H 2 O=2C 6 H 10 O5 



(Starch) 



