380 HOW CROPS GROW. 



vascular bundles, whether these are arranged symmetri- 

 cally and compactly, as in exogenous plants, or distrib- 

 uted singly through the stem, as in the endogens. This 

 is not only seen upon a bleeding stump, but is made evi- 

 dent by the oft-observed fact that colored liquids, when 

 absorbed into a plant or cutting, visibly follow the course 

 of the vessels, though they do not commonly penetrate 

 the spiral ducts, but ascend in the sieve-cells of the cam- 

 bium. * 



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 disssolved 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 

 diffusion, mostly through the walls of imperforate cells, 

 and goes on in all directions. Xew growth is the forma- 

 tion 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 communicate with each other by pores, their ex- 

 pansion has ceased. Henceforth they merely become 

 thickened by interior 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 exo- 

 genous tree, led to the erroneous assumption of a form- 

 al downward 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 



* As in T'nger's experiment of placing a hyacinth in the iuiee of the 

 poke weed < Phytolacca\ or in Ilallicr's observations on cuttings dipped 

 in cherry-juice. (.;*. .., IX, p. 1.; 



