568 



HOW CROPS GROW. 



If a twig be cut from a tree in winter 

 and be placed in a room having a summer 

 temperature, the buds, before dormant, 

 shortly exhibit signs of growth, and if 

 the cut end be immersed in water, the 

 buds will enlarge quite after the normal 

 manner, as long as the nutrient matters 

 of the twig last, or until the tissues at 

 the cut begin to decay. It is the summer 

 temperature which excites the chemical 

 changes that result in growth. Water 

 is needful to occupy the expanding and 

 new-forming cells, and to be the vehicle 

 for the translocation of nutrient matters 

 from the wood to the buds. Water en- 

 ters the cut stem by imbibition or capil- 

 larity, not merely enough to replace loss 

 by exhalation, but is sucked in by osmose 

 acting in the growing cells. Under the 

 same conditions as to temperature, the 

 twigs which are connected with active 

 roots expand earlier and more rapidly 

 than cuttings. Artificial pressure on the 

 water which is presented to the latter 

 acts with an effect similar to that which 

 the natural stress caused by the root- 

 power exerts. This fact was demon- 

 strated by Boehra {Sitzungsberichte der ^ 

 Wiener Akad., 1863) in an experiment 

 which may be made as illustrated by the 

 cut, fig. 70. A twig with buds is secured 

 by means of a perforated cork into one 

 end of a short, wide glass tube, which 

 is closed below by another cork through 

 which passes a narrow syphon-tube, £. 

 The cut end of the twis? is immersed in 



Fia;. 70. 



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