MOVEMENTS OF NUTKIENT MATTEES. 341 



vessels, though they do not commonly penetrate the spiral 

 ducts, but ascend in the sieve-cells of the cambium.* 



The rapid supply of water to the foliage of a plant, 

 either from the roots or from a vessel in which the cut 

 stem is immersed, goes on when the cellular tissues of the 

 bark and pith are removed or interrupted, but is at once 

 checked by severing the vascular bundles. 



The proper motion of the nutritive matters in the plant 

 of the salts dissolved from the soil and of the organic 

 principles compounded from carbonic acid, water, and 

 nitric acid or ammonia in the leaves is one of slow dif- 

 fusion mostly through the walls of imperforate cells, and 

 goes on in all directions. New growth is the formation 

 and expansion of new cells into which nutritive substances 

 are imbibed, but not poured through visible passages. 

 When closed cells are converted into ducts or visibly com- 

 municate with each other by pores, their expansion has 

 ceased. Henceforth they merely become thickened by in- 

 terior deposition. 



Movements of Nutrient Matters in the Bark or Rind. 

 The ancient observation of what ordinarily ensues when 

 a ring of bark is removed from the stem of an exogenous 

 tree, led to the erroneous assumption of a formal down- 

 ward current of " elaborated " sap in the bark. When a 

 cutting from one of our common trees is girdled at its 

 middle and then placed in circumstances favorable for 

 growth, as in moist, warm air, with its lower extremity in 

 water, roots form chiefly at the edge of the bark just 

 above the removed ring. The twisting, or half-breaking, 

 as well as ringing of a layer, promotes the development 

 of roots. Latent buds are often called forth on the stems 

 of fruit trees, and branches grow more vigorously, by 

 making a transverse incision through the bark* just below 



* As in Unger's experiment of placing a hyacinth in the juice of the poke- 

 weed (Phytdacca,) or in Hallier's observations on cuttings dipped in cherry -juice. 

 (Vs. St., IX, p. 1.) 



