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PHYSIOLOGY OF NUTRITION 



(Fig. 74). The tips of these are composed of spongy tissue and readily allow 

 the entrance of air. These outgrowths are like ventilation pipes in that they 

 promote the movement of air to the roots below. The air spaces of the cortex 

 are thus always directly or indirectly in communication with the external 

 atmosphere. 1 



The central woody cylinder of the stem also contains air. Hohnel's 2 exper- 

 iments indicate that the air in the wood and that in the cortex are not at all 

 continuous. In these experiments (Fig. 75) a leaf is fastened, by means of a 

 rubber stopper, in a wide-mouth bottle (g), which has a lateral opening below, 

 the latter fitted with a tube and funnel (/) through which mercury is introduced. 

 The air in the cylinder, compressed by the mercury, is forced through the sus- 

 pended leaf and rises in bubbles through the water in the glass vessel above (/). 



Fig. 74. — Part of stem of Jussiaa repens, with 

 ventilation roots (w) ; surface of water, O. 



Fig. 75. — Hohnel's apparatus. {After 

 Pfeffer.) 



Microscopic study shows that bubbles are extruded only from the cortex, not 

 from the central cylinder. Air enters the leaf through the stomata and air 

 spaces of the leaf cortex and is given out from the cortical region of the stem, 

 without entering the wood. The aeration system of the wood is a closed 

 system and does not communicate at all with that of the cortex. 



Hohnel demonstrated the existence of negative gas pressure in the wood of 

 stems. If a twig, or the petiole of a leaf, is cut under mercury on a sunny day 

 in summer, mercury rises very rapidly through the cut surface into the vessels 

 (Fig. 76), which then appear gray in cross-section, from the presence of the 



1 Goebel, K., Ueber die Luftwurzeln von Sonneratia. Ber. Deutsch. Bot. Ges. 4: 249-255. 1886. 

 Jost, Ludwig, Ein Beitrag zur Kenntniss der Athmungsorgane der Pflanzen. Bot. Zeitg. 45 : 601-606. 617- 



627, 633-642. 1887. 



2 Hohnel, Franz Xavier R. von, Einige anatomische Bemerkungen uber das raumhche Verhaltniss der 

 Intercellularraume zu den Gefassen. Bot. Zeitg. 37= 541-545- 187°. Idem, Beitrage zur Kenntniss der 

 Luft- und Saftbewegung in der Pflanze. Jahrb. wiss. Bot. 12 : 47-131- 1870- 1881. 



