16 PRINCIPLES AND PRACTICE OF PRUNING 



number of stomata in given areas, developing water-hold- 

 ing tissues, reducing the amount of surface exposed to 

 the air, and so on. Artificially, transpiration may be re- 

 duced by preventing the development of excessive leaf 

 area, as when nitrogenous plant food is withheld, and by 

 pruning off unnecessary branches and twigs and even in- 

 dividual leaves. 



Instances of water loss effects during dry spells are the 

 wilting and drying up of certain herbaceous annuals, the 

 premature defoliation of deciduous perennials, the partial 

 or total defoliation of fruit trees followed by the develop- 

 ment of new growth subsequent to a wet spell in early 

 autumn. Such instances indicate that short water supply 

 during the growing season tends to maturity of all parts, 

 especially fruits and seeds, the former of which are often 

 altered in texture and even chemical composition. 



20. Transportation of water and plant food. — In all 



plants with which this volume deals there are "vessels" 

 or conduits which greatly facilitate the transpiration of 

 liquids. These are of two classes. (1) Some of these, 

 the large and numerous xylem vessels (25) on the inner 

 side of the cambium, lead from the water-absorbing areas 

 to the transpiring areas. Through them the greater part 

 of the copious "transpiration stream" ascends. From 

 them also more or less water diffuses to cells which need 

 it, and which have enough tension to draw it. This 

 stream also carries mineral nutrients from part to part 

 upward. (2) Other of these vessels (sieve tubes), smaller 

 ones than those just mentioned, are outside the cambium. 

 They lead from the chlorophyll-containing parts of the 

 water-transpiring areas to all regions of the plant where 

 growth and food storage are taking place and pass by 

 diffusion through the cell walls where such activities are 

 occurring. These smaller conduits carry the "elaborated" 

 plant foods made from water and carbon dioxide in the 

 process of photosynthesis (29). 



