164 PHYSIOLOGY OF NUTRITION. 



closed with a lamella of cork, the mercury rises very slowly, e.g. 

 only 1 mm. per day. No rise at all takes place if we experiment 

 with dry pieces of Hedera Helix leaf. A fairly rapid rise of the 

 mercury is exhibited, if we lay on the pieces of tissue, after they 

 have been fixed, one end of a strip of blotting-paper the other end 

 of which dips into water, so that they imbibe water. It is found, 

 therefore, that gas dialysis, in which, of course, gas absorption 

 plays an important part, can only take place vigorously when the 

 membranes of the plant tissues are imbibed with water. Dry 

 membranes do not allow gases to dialyse, or they permit them 

 only very slow passage (periderm). Carbon dioxide traverses 

 plant membranes by dialysis more rapidly than Nitrogen and 

 Oxygen. 2 



1 See Borodin, Memoires de I'acad. imp. de St. Petersbourg, T. 28, No. 4. 



2 For the literature see Detmer, Lehrbuch d. Pflanzenphytiolngie, 1883, p. 97, 

 and Pfeffer, Handbuch d. Pflanzenphysiologie, Bd. 1, p. 86. Experimental 

 researches which are of interest have been carried out by N. J. C. Miiller (see 

 Pringsheim's Jahrbiicher, Bd. 7). Also see Wiesner, Sitzungsber. d. Akad. d. 

 Wiss. zu Wien, 1889, Bd. 98, Pt, 1, p. 693. 



65. The Intercellular System of Plants. 



The intercellular spaces of plants, which are so important in 

 connection with gas exchange, ventilation, and the hereafter-to- 

 be-considered formation of aqueous vapour in the plant, are found 

 particularly between the cells of the parenchyma, but also in other 

 places. They originate either schizogenously or lysigenously, and 

 in the latter case are often of considerable diameter. In the 

 parenchyma the intercellular spaces usually occur between the 

 rounded angles of the cells as triangular intercommunicating 

 canals. We first submit to microscopic examination delicate 

 transverse sections from the cotyledons of seeds of Lupinus luteus, 

 cut after soaking the seeds somewhat. We soon see the rather 

 narrow intercellular spaces between the cells. The air spaces are 

 naturally of special importance for normal germination, which is 

 associated with active gas exchange. The intercellular spaces are 

 also readily made out between the cells of the cotyledons of 

 Lupinus, when they have risen above ground and become green. 



We further prepare transverse sections through an internode 

 of Zea Mais (see Fig. 56). Air-dry material is quite suitable for 

 our purpose. The vascular bundles are not arranged in a circle, 



