396 LECTURE XXV. 



These statements are now to be supported more in detail by adducing the most 

 important facts ^. 



That the chemical processes and molecular movements which constitute the 

 growth of plants only take place when the atmospheric oxygen envelopes them and 

 is distributed throughout the organs, was first proved by the investigations of Theo- 

 dore de Saussure in 1804, with the caution and accuracy peculiar to this highly- 

 gifted experimenter. Dutrochet first showed, however,-that air containing oxygen 

 diffused in the tissues of the periodically motile and irritable organs is a condition of 

 their motility. On the sensitive leaves of a Mimosa standing, beneath the receiver 

 of the air-pump, exhaustion at first acted like a mechanical shock : in the vacuum, 

 however, they assume a permanent but rigid position. The periodic oscillations are 

 suppressed— they are not sensitive to shocks. The irritability and periodic move- 

 ments of the leaves return, however, when the plants are subsequently exposed to 

 the air again. In the same way, the periodically motile flowers of Leontodon taraxa- 

 cum and Sonchus oleraceus became fixed in Dutrochet's vacuum. According to later 

 researches by Kabsch, the stamens of Mahonia and Berheris, which are sensitive to 

 contact, are rendered rigid and cease to be irritable when the air. under the air-pump' 

 is much rarified : the same is the case with the stamens of Helianthemum vulgare. 

 On renewed access of air — i.e. of its oxygen — the motility of these organs again 

 returns. Kabsch showed that this is due merely to the oxygen, by allowing the 

 Organs mentioned to remain for some time in pure nitrogen. On again exposing 

 them to the atmosphere after 10-15 minutes, they regained their irritability ; while 

 they lost it for ever on remaining for a longer time in nitrogen. A stay in pure 

 hydrogen acted similarly. With respect to the indispensability of an atmosphere 

 containing oxygen for the maintenance of the streaming of the protoplasm in cells, 

 as well as in the naked protoplasm of the Myxomycetes, Kuhne published detailed 

 observations so long ago as 1864. This phenomenon also, according to his 

 observations, disappears on the exclusion of atmospheric air, but returns, if it had 

 not continued too long, after a few minutes on the access of ordinary air. 



I have given prominence to these facts, because they ^demonstrate directly and 

 without any need of comment the importance of oxygen respiration. As animals 

 are suffocated by withdrawal of atinospheric oxygen, so with plants also : their 

 functions come to a standstill, and if the respiration is not restored at the right 

 time, the standstill in permanent, and death' results. It is true, as we shall see 

 later, the respiration of plants is far less energetic than that of warm-blooded 

 animals, but probably it can be compared in all respects with that of cold-blooded 

 animals. 



der Exp. Phys.,' the path to a correct understanding of the respiration of plants, lost for twenty-fivs; 

 years, was first recovered. Numerous more special investigations have since appeared, without, 

 however, essentially altering the matter itself; only the so-called intra-molecular respiration, occurring 

 in plants just as in animals, can be regarded as an important addition to the facts long known. 



1 What here follows in the text is essentially a short extract from the chapter ' Die Athmmg 

 der Pflanzen, Wdrmebildung und Phosphorescence in my 'Exp. Phys.' (1865, pp. 263-304)- The 

 more recent literature is collected in Pfeffer's ' PJlanzenphysiologie' Of the newer works, Borodin, 

 ' Sur la Respiration des Plantis ' (Florence, 1875), and ' Untersuchungen iiber die Pflanzenathmung' 

 in Mem. de I'acad. imp. des sc. de St. Petersbourg, VII» Sdr. t. XXVII, No. 4, 1881, are to be 

 mentioned. 



