174 THE PLANT CELL. 



ground-air into ammonia, nitrites, and nitrates. The latter 

 salts are then absorbed by the roots.* The bacteria are known 

 as "nitrifying bacteria," one of the forms being Closterium 

 Pasteurianum,. 



In Chap. iii. it was mentioned that certain glandular capitate 

 hairs were able to absorb ammonia from the atmosphere. It 

 should be understood, however, that it is most probably ammo- 

 nium nitrite which is absorbed, since this substance exists at times 

 in the air {cf. Thorpe's Inorganic Chemistry, vol. i.). The capacity 

 of working up nitrogen possessed by the different species of soil 

 bacteria has been put to practical test of recent years in con- 

 nection with the raisihg of cereals. 



The manner in which the dilute solution of salts, or raw sap, 

 is drawn up through the vascular tissues of the root and stem 

 until it finally reaches the leaves of a plant must next be 

 examined, and in this connection it is necessary to consider two 

 phenomena. The first of these is the transpiration current, 

 and the second root-pressure. 



The transpiration current is an upward flow of sap through 

 the wood of root and stem, caused primarily by tlie suction 

 action produced by the evaporation of water from the leaves. 

 It has been found that about 98 per cent, of the radiant energy 

 absorbed by a plant is utilised in evaporating the water of 

 transpiration. Tliis evaporation takes place through the 

 stomata, the mesophyll cells surrounding the respiratory cavity 

 of each stoma, constantly giving off, during the daytime, 

 moisture, which collects in and is subsequently evaporated 

 from the respiratory chamber. The effect of this loss of water 

 from some of the mesophyll cells is to draw in water by osmosis 

 from cells of the spongy parenchyma, which are more remote, 

 and ultimately from the annular and spiral elements of the 

 leaf- traces, which, as has been seen, lie in the mesophyll of a 

 bifacial leaf; and since these elements are the terminations of 

 the fibrovascular bundles of the petioles and ultimately of the 

 stem, water is being constantly sucked up from elements 

 containing sap of progressively decreasing concentration (see 

 Fig. 113). 



In this manner a current — the so-called transpiration current 



* See Muir and Ritchie, Manual of Bacteriology, 1907 ; also Detmer 

 .and Moore, Practical Plant Physiology. 



