BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 287 



they have no special organs of respiration comparable to the lungs of 

 animals. Yet special contrivances exist which facilitate the passage 

 of oxygen from the atmosphere to every part of the plant. Inter- 

 cellular passages penetrating the tissues communicate externally with 

 the stomates, and with larger pores in the bark, called lenticels. Len- 

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

 upon one another, and over which the epidermis is broken away. They 

 may be seen upon almost any twig. The intercellular spaces of water 

 plants are particularly large in order to convey to submerged parts the 

 oxygen 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 through the tissues dissolved in the liquids of the cells, 

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

 needed. 



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

 gen may best be seen in those cells ^ in which the protoplasm streams, 

 — that is, circulates in the cell more or less rapidly (Fig. 360). If 

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

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

 watched under the microscope, the movement is seen to lessen within 

 a few seconds after oxygen is driven off, and shortly to stop altogether. 

 If, after not too long a time, oxygen is once more admitted, the stream- 

 ing of the protoplasm begins again. But if the suspense is too long, 

 the protoplasm will be found to be dead. 



568. In respiration, the oxygen absorbed by the protoplasm slowly 

 oxidizes it. There is, in other words, a slow burning. Of course the 

 protoplasm is slowly destroyed, and has to be renewed through nutri- 

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

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

 process is essentially like that by which 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 the oxidizing process carbonic acid gas is formed. This 

 gas is easy to detect experimentally,^ and when given off by the plant 

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

 continuously, as long as they live. But in daytime respiration is not 

 easy to show, since the carbon dioxide given up by the respiring cells 

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

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

 telltale gas. 



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

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

 trichomes of many plants. 



2 See Experiment 12, p. 60. 



