32 PLANT PHYSIOLOGY 



we may conclude that the plant requires in round number 400 g. per month 

 of C0 , assuming a daily assimilatory period of ten hours. If we now imagine 

 the 



1. 49, for 135 billion kg. read 54 billion kg. 



119, 11. 1-9, for twenty months. This is ... exact analyses read fifty 

 months. It would last somewhat longer if we took as our basis the C0 2 require- 

 ments of a wood. In any case it would appear that the supply of C0 2 in the 

 air must in the course of a few years be entirely used up by the activity of 

 green plants. Since, as a matter of fact, no decrease in the amount of CO 2 in 

 the air has, according to the most exact analyses, 



I. 21, for 279 read 298 



II. 32-3, for everywhere . . . present, read everywhere, in round numbers, 

 0-03 per cent, present. 



1. 39, after 20 C. read Water at 15 C. in contact with the air has approxi- 

 mately the same amount of CO 2 dissolved in it, viz. 0-03 per cent. 



1. 50 P. 120, 1. 37, for We owe our knowledge . . . though water cannot 

 read To GODLEWSKI'S (1873) and KREUSSLER'S (1885) investigations, as also to 

 those of BROWN and ESCOMBE (1902), PANTANELLI (1903), and of BLACKMAN 

 and MATTHAEI (1905), we owe our knowledge of the fact that an increase in 

 the amount of CO 2 in the air tends to an increase in assimilation. On the 

 other hand, it is well known that a higher percentage of C0 2 induces closure 

 of the stomata (DARWIN, 1898), and hence injures the plant, while a higher 

 percentage still acts directly as a poison and destroys vitality (LopRiORi, 

 1895). Hence there must be a certain medium percentage which induces an 

 optimum effect. This optimum is not, however, constant, for the assimilatory 

 activity depends on several other factors, e.g. light and temperature. Thus 

 it arises that a percentage of CO 2 in the air which permits a maximum of assimila- 

 tion when the light has a certain intensity, becomes a limiting factor at higher 

 light intensities (BLACKMAN, 1905). 



Let us now turn to the question how the carbon-dioxide reaches the 

 assimilating cells of the leaf. Submerged plants are restricted entirely to 

 carbon-dioxide, and gases generally, dissolved in water, and these gases manage 

 to enter the plant by diffusion only through the continuous epidermis. Once 

 the gases have penetrated the outer wall of the epidermis they can diffuse 

 from cell to cell ; moreover, they can also pass through the inner walls of the 

 epidermal cells into the intercellular spaces, which are always exceedingly 

 plentiful in aquatic plants, and can thus enter the individual cells from these 

 spaces. Diffusion through the epidermal cells into the intercellular spaces 

 takes place (DEVAUX, 1889) in accordance with the same law that EXNER 

 established for the diffusion of gases through an aqueous film, i.e. the rate 

 of diffusion of the gas is directly proportional to its solubility in water, and 

 inversely proportional to the square root of the density of the gas. Hence it 

 follows that the rate of diffusion of oxygen is twice, and of carbon-dioxide 

 55 times, as great as that of nitrogen. When the diffusion movement has pro- 

 duced an equilibrium, the intercellular spaces are found to contain air of 

 approximately the same composition and at the same pressure as the atmo- 

 sphere. Respiration (Lecture XVI) induces no essential alteration in this 

 condition, but it is otherwise with carbon assimilation. Since the CO 2 streams 

 rapidly from without inwards, in proportion as it is used up, while the oxygen 

 arising from its decomposition diffuses outwards but slowly, increase of pres- 

 sure results in the intercellular spaces, and finally an extrusion of air-bubbles 

 from wounds produced intentionally or accidentally. This stream of air- 

 bubbles we have already observed and used as a measure of (^-decompo- 

 sition. That the air given off cannot be pure oxygen, but only a gaseous 



