158 



All Food Is Made by Green Flams unit hi 



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Fig. 185 The hickory leaflet (part of a compound leaf) was treated to remove all 

 tissues except the veins and epidermis. The dark spots are diseased areas. The smallest 

 veins co?2taiji both xylem and phloem. They are connected through the larger vein 

 and the leafstalk to similar tissues iit the stem and roots, (hugh spencer) 



manufactured products and some min- 

 erals from the soil travel through the sieve 

 tubes of the phloem. Sugar made in the 

 leaves may move down through the sieve 

 tubes to the roots in large amounts. It 

 may accumulate there as sugar or starch 

 and form a thickened, fleshy root. Or the 

 sugar may be combined with minerals 

 into protein; this can happen in any part 

 of the plant. Passage of sugar and minerals 

 through sieve tubes may be up or down. 

 But proteins, fats, and starches cannot 

 move by diffusion since thev^ are insol- 

 uble and the protoplasm of the cell is not 

 permeable to them. In general, they are 

 manufactured within the cell in which 

 they are found. All three, however, may 

 be made soluble (digested) in the cells 

 and the soluble products ma\' then move 

 to other parts of the plant. 



Digestion of insoluble foods in plants. 

 Most of us are quite aware that digestion 

 of insoluble foods occurs in animals. We 

 have a digestive system (see page 1S8) in 

 which insoluble foods are made soluble. 



Plants, too, have digestion although they 

 have no digestive system. During the 

 daytime starch accumulates in the leaves 

 and green stems of plants. Much of this 

 accumulated starch is digested to sugar 

 which moves to other parts of the plant 

 durintr the night. In temperate climates, 

 starch, protein, and fats accumulate in 

 stems, roots, and seeds in the summer. 

 This insoluble food is dio-ested the fol- 

 lowing spring and the soluble products 

 are used in tiX new growth of young 

 stems, leaves, roots, and seedlings. You 

 will study about digestion in the next 

 unit. 



The forces that move water in plants. 

 Water and mineral compounds enter the 

 epidermis of the root by diffusion, largely 

 through the root hairs. They pass by 

 diffusion across the cortex and outer 

 part of the vascular cylinder from cell 

 to cell. Here the water enters the xylem 

 tubes. As long as the water supply lasts 

 in the soil, more and more water diffuses 

 into the xylem tubes. The water then 



