56 PRINCIPLES OF PLANT CULTURE 



In such cases the roots perish, and the tree is found dead 

 the following spring. This most frequently occurs with 

 trees on poor soil, that have suffered from insect attacks 

 as well as from a dearth of water. It often occurs also 

 in recently-transplanted trees that fail to make satis- 

 factory growth the first season. 



82. Weed destruction. — To destroy the most persist- 

 ent weeds we starve the roots by preventing all leaf 

 growth (339). 



83. Restriction of the descending food current promotes 

 fruitfulness by causing an accumulation of prepared food 

 in the stem and branches (134). This is a weakening 

 process, however, and should not be extensively practiced 

 under cultural conditions. 



84. The storage of reserve food. — In healthy plants 

 food is usually prepared faster than it is consumed by 

 growth. The surplus may be in the form of starch, 

 as in the potato (Fig. 16), wheat and sago; sugar, as 

 in the sugar cane, sugar maple and beet; or oil, as in 

 cotton seed, flax seed and rape. Aside from the seeds, 

 which are always stocked with reserve food, certain 

 plants living more than one year, as the potato, beet, 

 onion, kohlrabi, and the like, have special accumulations 

 of food in certain parts, and the parts of plants that con- 

 tain such reserve food are most valuable as food for man 

 or animals. The proportion of starch stored in potato 

 tubers is not constant, hence the food value of different 

 samples of potatoes may vary greatly. In woody plants 

 the surplus food is more evenly distributed through the 

 different parts, though the older leaf -bearing wood is 

 usually best supplied. 



85. Use of reserve food. — Plants use their reserve 

 food in the production of flowers and seeds (134), and in 



