138 THE PHYSIOLOGY OF PLANTS 



generally that winter-pruning is strengthening, while summer- 

 pruning is weakening. 



If any portion of the shoot system is taken away after it 

 ha,s passed through one summer,- the structure and activity of 

 the root system — that is, its power of absorption and of forcing 

 up water — is such that it can nourish all the branches^ At 

 the beginning of the next period of activity, by cutting away 

 some branches the water- consuming area is diminished. The 

 same amount of pressure has therefore a reduced ' field of 

 action, and consequently the effect on the remaining branches 

 must be increased. 



By pruning in the summer we remove soft shoots with 

 only recently developed leaves. The latter have yet their chief 

 work to perform. For at the commencement they are de- 

 veloped at the cost of the reserve material which is stored up 

 in the branch ; then comes a period at which the young leaf 

 requires all the substance it assimilates from without for its 

 own growth, and only after its full development does it begin 

 to work for the benefit of the branch. If, therefore, a soft 

 shoot is taken away, the older portions of the branch are robbed 

 of the materials which were used in the unfolding of the leaves, 

 without receiving anything in return from the leaves they have 

 developed. This causes, therefore, a loss to the general economy 

 of the plant; but, with the increased productiveness of our 

 cultivated plants, sucih a slight weakening may be overlooked, 

 if any othef special advantage is gained. 



The staking away of tips of young, shoots, which is called 

 pinching, is admissible in trees which are unproductive on 

 account of the excessive development of shoOts. If we take 

 away the tip of a long shoot, we deprive it of the part which 

 makes the greatest demand upon its nutritive powers ; conse- 

 quently the pressure is increased in the lateral buds, the lowest 

 of which is increased in size while the upper ones will, soon 

 grow out to new shoots. The cells of the lower buds which 

 are thus enlarged will store up a larger quantity of reserve 

 material, and this must be present if flower-buds are to be 

 formed. 



The greatest success will attend this process if the pinching 

 takes place just at the period when the buds have still suffi- 



