1 86 



Journal of Agrkidfiire. Victoria. [lo March. 1912. 



guard cells joined together at the extremities in such a manner as to leave 

 a very narrow slit-]ike pore between them. The opening and closing of 

 these stomata are brought about by changes in the curvature o£ the guard 

 cells, and this in its turn is dependent on the turgiditv or water content 

 of the cells- 



Tt is interesting to note that in this specimen of wlieat the number of 

 stomata on the uffer surface per square centimetre amounted to 3,681, 

 whilst the number on the lozver surface was 3.321 per square cm. 



Respiration. — These stomato are the breathing pores of the plant. Re- 

 spiration is quite as necessary for fhe life process of plants as it is for 

 animals and the process is essentially the same in animals. The stems, 

 roots and leaves of a plant are constantly in need of oxygen for respiration, 

 and while under ordinary conditions the aerial portions of a plant can 

 secure a sufficiency of oxygen for their requirements, it not infrequently 

 happens that the roots suffer from lack of proper supplies of air in the 



soil. This may readily be 

 seen in the unhealthy, sickly 

 yellow appearance of wheat 

 growing in low-lying por- 

 tions of fields after heavy 

 winter rains, or in badly 

 drained water-logged soils. 

 Respiration goes on through- 

 out a plant's existence and 

 its general effect is to destroy 

 the carbonaceous material of 

 the plant and liberate car- 

 bonic acid gas. 



Assimilation or Phoiosyn- 

 tlicsis. — This process of re- 

 spiration in green plants is 

 overshadowed by the oppo- 

 site process of assimilation. 

 The leaf is the medium 

 ' whereby all green plants are 



13. TRANSVERSE SECTION OF THE STEM OF enabled to obtain from the air 

 FEDERATION WHEAT, SHOWING VASCULAR ^^e carbon which forms the 

 BUNDLES. greater portion of their bulk. 



Carbonic acid gas is present in ordinary air to the extent of about four parts 

 in 10,000. Air containing carbonic acid enters the stomata and circulates 

 freelv in the intercellular spaces. Under the influence of sunlight which 

 supplies the necessarv energy and in the presence of the chlorophyll and 

 moisture in the leaf, the carbon of the carbonic acid is fixed by being 

 converted into carbohydrates and oxygen is set free and exhaled by the 

 plant. This process is called " carbon fixation," " assmiilation," or as 

 it is dependent on light " photosynthesis." The energy necessary to effect 

 these changes is of course obtained from the light and the leaf has trans- 

 formed the energy of sunlight into potential energy possessed by the newly 

 formed carbohydrate. Stephenson for this reason described coal as 

 "bottled sunlight," because the energy it possessed was derived from the 

 sunlight, which, ages ago assisted to groA> the vegetation which we now 

 mine as coal. 



