32 TRANSPIRATION AND ASCENT OF SAP ch. 



as it revealed no direct air-passages upon trial. As the 

 hematoxylin does not stain wood, this experiment points 

 to a mechanical separation from the gelatine owing to the 

 passage of the latter through membranes or walls. It is 

 possible, however, that some of the stain was taken up by 

 the cellulose walls of the medullary rays and the tori of 

 the pit membranes. 



Microscopical examination of branches choked with gela- 

 tine mixed with Indian ink, after the manner of Errera 

 and Strasburger, showed that the closing membranes of 

 the pit had exerted a straining action, accumulating Indian 

 ink upon the one side, so that the pits were picked out 

 very sharply as black objects. This filtering action is 

 suggestive of the passage of the medium carrying the 

 precipitate ; and although, so far as this observation 

 is concerned, there might have been nitration of the 

 gelatine from the water in which it was dissolved, still, 

 taken in conjunction with the other observations, we 

 think it supports the view led to by those observations, i.e., 

 that dilute melted gelatine can pass through the substance 

 of the closing membranes, and, if so, is very probably 

 capable of penetrating into the cell-wall, or otherwise we 

 must suppose perforations to exist in the pit-membrane 

 or its torus. 



Effects of paraffin and gelatine compared. 

 The effect of using paraffin wax of low melting-point as 

 the material for choking the lumina was also tried and 

 compared with that of gelatine. Four similar branches of 

 lime, Tilia europaea, were cut (May 9), and put standing 

 for twenty minutes in water at 50 C, immersed to a 

 depth of about 20 cm. These were called A, B, C, D. 



A was preserved in water at 50. 



B was transferred to melted paraffin at 50 (melting- 

 point 48). 



C was transferred to gelatine coloured with Indian ink 

 at 50. 



