314 



Wiesner^ made further observations on gas exchange. He shows that 

 the periderm, the cork covering, is completely impermeable to air even with 

 great differences in pressure. The exchange takes place only through the 

 lenticels which are permeable even in winter. In wood free from ducts the 

 equalization takes place through the cell walls, especially through the deli- 

 cate pitted walls in which, besides the diffusion, absorption through the col- 

 loidal walls comes into effect. In woody bodies, rich in ducts, transpiration 

 and the penetration of gases through the ducts, functioning as capillary 

 tubes, should also be taken into consideration. The equalization of the 

 pressure takes place more quickly axially than transversely. The more 

 turgid a parenchyma or wood cell is, the more slowly does the equalization 

 of the pressure occur. This relation is reversed in the periderm cell. If it 

 incurs the loss of its aqueous contents and is filled with air, whereby its wall 

 becomes dry, the cell loses its permeability for gases. In parenchyma 

 which conducts air, a part of the air flows through the intercellular passages 

 during the equalization of the pressure, another part passes through the 

 closed membranes and, indeed, most easily through the places which have 

 remained unthickened. 



A statement by Mangin- throws light on the processes taking place in 

 trees, with poor soil aeration. He found that the ducts in Ailanthus were 

 filled with tyloses, and, in explaining the process, states that, correlative with 

 a lack of air in the soil, a deficiency in the supply of air in the ducts takes 

 place. Consequently the air in the ducts becomes diluted beyond the opti- 

 mum and the tyloses of the adjacent cells push into the tube of the duct and, 

 on their part, also hinder the conducting of water. 



In regard to the influence of a lack of oxygen on seeds, Bert's^ investi- 

 gations should be considered first of all, according to which germination 

 progresses more slowly in a lesser air pressure. Many years ago Cofti* 

 observed that a dilution of the air had an arresting influence on the cyto- 

 plasmic currents. Since, however, with a normal air pressure and only de- 

 creased oxygen content, germination takes place more slowly and, con- 

 versely, with a lowered air pressure but increased supply of oxygen the 

 seeds germinate more rapidly, it is evident that even the partial pressure of 

 the oxygen alone is a decisive factor. 



In the phenomena due to lack of oxygen, opportunity is again offered 

 of pointing to the fact that sudden changes are more disturbing than gradual 

 changes. Stich^ found that in an atmosphere poor in oxygen the normal 

 respiratory quotient is recovered by decreasing the absolute amounts of oxy- 



1 Wiesner, Versuch iiber den Ausgleich des Gasdruckes in den Geweben der 

 Pflanzen. Sitz. d. Kais. Akad. d. Wissensch. zu Wien am 17 April, cit. in Oesterr. 

 Bot. Zeit. 1879, p. 202. 



2 Mangin, Influence de la rarefaction produite dans la tige sur la formation 

 des thylles gommeuses. Compt. rend, 1901, II, p. 305. 



3 Bert, Recherches experimentales sur I'influence que les changements dans la 

 pression barometrique exercent sur les phenomenes de la vie. Compt. rend LXXVI 

 et LXXVII. 



4 Meyen, Pflanzenphysiologie, 1838, II, p. 224. 



5 Stich, C, Die Atmung der Pflanzen bei verminderter Sauerstoffspannung 

 und bei Verletzungen. Flora, 1891, p. 1. 



