PASSAGE-CELLS. FUNCTION OF CASPARIAN STRIP 371 



(Fig. 148). In tangential section they are seen to be scattered in 

 certain cases, while in other instances they form longitudinal series. 



Schwendener has shown by experiment, that the passage-cells do 

 actually represent permeable spots in an otherwise impervious enclo- 

 dermis ; he has also drawn certain conclusions from his experiments 

 concerning the physiological significance of these structures, and 

 especially as regards their topographical relation to the vessels. In 

 view of the fact that the vessels are water-conducting tubes, the 

 passage-cells must undoubtedly serve, in Schwendener's opinion, " to 

 establish direct communication between the water-conducting channels 

 and the living cortex, thus corresponding, as it were, to the open 

 side-sluices of an extensive system of irrigating canals, the main 

 channels of which are represented by the large vessels." 



In stems and foliar organs the passage-cells are arranged on a 

 similar plan. A deviation from the ordinary arrangement occurs in the 

 leaves of many Orchidaceae and Bromeliaceae, where the passage- 

 cells, instead of being placed on the inner face of the collateral bundle 

 opposite the annular and spiral protoxylem vessels, are situated on the 

 flanks of the bundle, either exactly along the line of junction of 

 hadrome and leptome, or more often rather nearer the inner margin of 

 the leptome. As Schwendener remarks, this unusual position clearly 

 suggests that, in such cases, the passage-cells stand in close functional 

 relation to the leptome as well as to the hadrome. 



That an endodermis which has both its tangential and its radial 

 walls suberised, must be relatively impervious, is self-evident. Different 

 conditions, however, prevail, where the tangential walls are entirely 

 unsuberised, while in the case of the radial septa suberisation is 

 restricted to the above-mentioned narrow strip. Opinions differ con- 

 siderably as to the physiological significance of the Casparian strips. 

 Collectively they form a tubular mesh work, to which Schwendener 

 ascribes a purely mechanical function ; in this investigator's opinion, in 

 fact, the endodermis may be compared to the protective net- work of ropes 

 in which a balloon is encased. According to Strasburger, the suberised 

 strips of the radial walls perform a mechanical function of a different 

 nature, by increasing the lateral cohesion between adjacent cells of the 

 endodermis, and thus preventing the formation of radial intercellular 

 spaces within that layer. By this means the ventilating system of the 

 cortex is permanently shut off from that of the central cylinder, with 

 the result that considerable negative pressures can be maintained in 

 the water-conducting channels. It should be noted, that De Vries had 

 previously drawn attention to the importance of the endodermis as a 

 pressure-boundary between cortex and central cylinder, in the case of 

 roots, and had also shown that when water is injected under high 



