BlUEF OUTLINE OF VFMETAULE PHYSIOLOGY 2:J7 



tlipy liave no special organs of respiration comparable to the lungs of 

 animals. Yet special contrivances exist wliicli facilitate the passage 

 of oxygen from the atmosphere to every part of the i>lant. Inter- 

 cellnlar passages penetrating the tissues communicate externally with 

 the stomates, and with larger pores in the bark, called Icniiceh. Leii- 

 ticels are slight outgrowths of the cork, in wliich the cells lie loosely 

 upon one another, and over which the epidei'uiis is broken away. 'J'hey 

 maybe seen upon almost any twig. The intercelhdar spaces of water 

 phmts are particularly large in order to convey to sulnnerged pai'ts the 

 iixygen taken in through the .stomates of the leaf; or at least in order 

 to retain the oxygen given off by assimilating cells. Oxygen also 

 travels througli the tissues dissolved in the liquids of the cells, 

 by ordinary diffusion. In solution it enters the ceil where it is 

 needed. 



567. All living cells require oxygen. The effect of excluding oxy- 

 gen may best be seen in those cells' in wliich the pidloiilasni streams, 

 — that is, circulates in the cell more or less rapiidly (Fig. -jtiU). If 

 arrangements are made to supply some other gas — as carbon ili- 

 oxide — to the cell while the circulation of the piotmilasm is being 

 watched under the microscope, the movement is seen to li'ssen williin 

 a few seconds after oxygen is driven oif, and shortly to stup altouetliei'. 

 If, after not too long a time, oxygen is once more admitti'd, the si leani- 

 ing of the protoplasm begins again. l!ut it the suspense is tou long, 

 the protoplasm will be foiunl to be dead. 



568. In respii'ation, the oxygen absorlied by the protoplasm slowly 

 oxidizes it. There is, in other words, a slow burning. Of ctmrse tlie 

 protoplasm is slowly destroyed, and has to be renewed througli nuti'i- 

 tion. The result of oxidation, however, is the generation of heat and 

 other forms of energy, which enable the cells to do their \York. The 

 process is essentially like that by wdiich energy is "set free" in the 

 burning of coal for the driving of an engine. All engines, whether 

 organic or inorganic, consume fuel. 



569. By tlie oxidizing process cai-boiiic acid gas is formed. This 

 gas is easy to detect experimentally,^ and when given otf by the pilant 

 furnishes the best evidence that respiration is going on. Plants respire 

 conliiiuonsly, as long as they live. But in daytime respiiration is not 

 easy to show, since the carbon dioxide given up by the re.spiring cells 

 is taken in by the assimilatory tissues. At night or in darkness, on 

 the other hand, respiration is clearly indicated by the escapie of the 

 tellttde gas. 



1 Such as the new root hairs of some aquatics, the cells of the leaf of 

 the tresh-water Eelgrass, and cells of the alga called Chara, and young 

 crichomes of many plants. 



2 See Experiment 12, p. 00. 



