ORGANIC SUBSTANCES AS PLANT FOOD 245 



flowering and the maturing of the grain, notwithstanding the rapid 

 increase in weight of the grain, which in the end contains almost 

 two-fifths of the total dry matter; hence, it is apparent that the 

 weight of the remaining organs of the plant must have decreased 

 by the same amount. 



The substances that are removed from the leaves are distributed 

 throughout the plant. Part of them are utilized by the growing 

 regions, the young developing stems and leaves and the tips of 

 roots. Another fraction is directed into the ripening fruits and 

 seeds, or the underground storage organs, or the living tissues of 

 the trunk of the tree, where it is stored as a food reserve. The 

 storage processes are exactly the reverse of those which take place 

 at germination, from the incoming sugars and amino acids there 

 are formed polysaccharides, mainly starch, fats, and reserve pro- 

 teins. The chemistry of the synthetic processes is not nearly so 

 well known as the chemistry of hydrolysis of these reserve sub- 

 stances. It should be noted that in all of these processes water is 

 given off. Hence, in general, the maturation of seeds and the 

 ripening of the wood in the fall in preparation for dormancy, pro- 

 ceed more rapidly in dry weather, when water is being continually 

 removed. An important role in synthesis during food storage is 

 apparently played by enzymes, the action of which, as we saw in 

 Art. 68, is reversible. One may, therefore, suppose that the syn- 

 thesis of polysaccharides, fats, and proteins is brought about by the 

 activity of the same carbohydrases, lipases, and proteases which 

 bring about their hydrolysis at germination. 



Nutritive substances move largely through the sieve tubes 

 and, in general the phloem parts of the conducting vessels. 

 The most evident proof of this has been furnished by ringing 

 experiments with branches or even whole tree trunks. In woody 

 plants there is a solid ring of xylem, through which water moves, 

 and on the outside of it a solid ring of phloem, composing the con- 

 ducting part of the bark. If a branch of willow is ringed by cut- 

 ting and removing the bark 1 to 2 cm. in width, and then placed 

 in water, so that the ringed portion will remain in damp air, it will 

 be seen that the adventitious roots develop considerably better 

 and in greater number above the ringed portion (Fig. 98), while the 

 control branch will have most of its roots at the very base of the 

 branch. If a branch is ringed on the tree, the bark at the upper 

 edge of the cut will begin to thicken more, forming a ringlike 



