FUNDAMENTAL PRINCIPLES OF PLANT GROWTH. 21 



ing readily, girdling is sometimes practiced in order to throw 

 the plant into bearing. This may be done in a variety of ways 

 but should not be attempted except in the case of trees that 

 seem hopeless otherwise. 



Girdling may be done successfully as follows: 

 With an ordinary cross-cut saw, cut in a spiral direction 

 around the trunk or branch to be treated and have th-e cut end 

 just under where it commenced but several inches below. In 

 this way the circulation of sap is only sufficiently impeded to 

 cause flower buds to form. Such wounds seldom cause serious 

 injury to vigorous trees. This work should be done in June if 

 at all, but is seldom a desirable practice. 



The leaves of plants are made up of 

 loose, open tissue enclosed in a thin mem- 

 brane. This membrane has openings in it 

 through which the plant takes in carbonic 

 acid gas, i. e. carbon dioxide from the air, 

 and throws off large quantities of water. 

 It Is through these openings, called stomata, 

 that diseases frequently enter the plant. 

 Such openings also occur in the young 

 twigs of some plants. It is in its green por- 

 tions that the plant absorbs and assimilates 

 food, and since this green portion is formed 

 almost exclusively in our fruit plants only 

 in the presence of direct sunlight, the im- 

 portance of lots of sunlight for best develop- 

 ment of these plants is evident. 



The flower is the portion of the tree 

 designed for the production of seed. All 

 the parts of our fruit plants in a natural 

 state seem to facilitate this object. Flow- 

 ers are often imperfect, as in the case ot 

 some forest trees, but in our com- 

 mon cultivated fruit plants the flowers are generally perfect 

 and only such flowers are referred to here. But flowers that are 

 perfect may be self-sterile, that is, may not be fertile to their 

 own pollen but need to ba cross fertilized. 



Fig. 2. — Trunk of 

 apple tree two 

 inches in diam- 

 eter one year 

 after girdling 



with saw. 



