SUPPLEMENT 33 



mixture rich in that gas is readily understood, now that we have become 

 acquainted with diffusion more in detail. The nitrogen diffuses continuously 

 out of the water into the intercellular spaces, and all the more so the richer 

 these become in oxygen owing to the decomposition of CO 2 . Were the rate 

 of diffusion not so slow the escaping gas would always exhibit the same pro- 

 portion between oxygen and nitrogen as that which exists in air. 



There is another reason for the evolution of air-bubbles from aquatic 

 plants, which must be taken note of because it not infrequently may give rise 

 to misconception. One notices often when using the bubble method for demon- 

 stration purposes that the stream does not cease in darkness. This is always 

 the case when the water employed is previously supersaturated with gases, 

 or when it becomes so by a rise in temperature. Under these circumstances, 

 the surplus gas must come out of solution and air-bubbles appear on the 

 walls of the vessel and on the outer surface of the plants ; it diffuses also in 

 bulk into the interior of the plant, causes a super-pressure there, and con- 

 sequently a stream of bubbles to the exterior (DEVAUX, 1889 ; PANTANELLI, 

 1904). It is advisable, therefore, in all experiments in assimilation to employ 

 water which has stood exposed, and not water direct from a tap. 



The absorption of carbon- dioxide by the leaves of land plants is carried 

 out in an essentially different manner from that seen in aquatic plants. In 

 land plants the epidermis is covered by a corky layer the cuticle which is 

 almost impermeable to water, although by no means so to carbon-dioxide ; 

 indeed carbon-dioxide can diffuse through a layer of oil, though water cannot. 



121, 1. 30, for 1-8 g. read 0-5 g. 



1. 49, after opening read (BROWN and ESCOMBE'S experiments have been 

 re-examined from the physical point of view by P. NELL (1905), but his results 

 are by no means entirely confirmatory) . 



122, 1. 12, for 0-134 ccm. read 0-447 ccm - 



I. 13, for 1-8 g. read 0-6 g. ; for 6 per cent, read 2 per cent. 



II. 46-8, for in carbon-dioxide, and it must . . . Assimilation is not 

 lowered read in carbon-dioxide, for assimilation is not lowered 



11. 52-6, for It must not . . . water present read This, however, is by 

 no means universally true, for a reduction in osmotic pressure produces an 

 unfavourable effect on assimilation usually long before the plasmolytic stage 

 is reached (PANTANELLI, 1903 ; TREBOUX, 1903). In addition to light and 

 atmospheric moisture there are other factors still which affect the behaviour 

 of the stomata. We need only refer to the carbon-dioxide itself, which, accord- 

 ing to DARWIN (1898), induces closure of the stoma. DARWIN unfortunately 

 does not mention the concentration of CO 2 which leads to this result. 



123, 1. 18, read For the formation of chlorophyll the temperature must 

 not be 



11. 23-43, delete Even when . . . (comp. p. 114). 



I. 44, delete only 



II. 48-50, for between such . . . indirectly read between external con- 

 ditions as influencing this process directly and as influencing it indirectly. 



11. 51-2, for inhibit the action of the chlorophyll read bring about a 

 closure of the stomata, 



124, 11. 1-2, for it cannot be replaced . . . carbon-monoxide read it cannot 

 apparently be replaced by any other compound of carbon. The compound 

 which would first occur to us is carbon-monoxide, and from JUST'S researches 

 (1882) it was supposed that this gas was indeed innocuous, but at the same 

 time useless. More recently, quite contradictory statements have been made 

 on the subject ; on the one hand it has been asserted that CO might serve 



JOST c 



