K.— BOTANY. ' 199 



may be, remain filled with colourless liquid, presumably the sap drawn 

 upward to supply the transpiring surfaces of the leaf. Generally the 

 coloured liquid descends an appreciable distance in the tracheae of the 

 stem before it begins to rise in the ascending current, mounting to other 

 transpiring leaves. As a rule after some time — depending on the rate 

 of transpiration and the amount of water supplied by the i-oots— the 

 presence of the coloured liquid may be demonstrated in certain con- 

 tinuous series, or filaments of trachese in several bundles of the lower 

 parts of the stems. Similarly, if tubers or rhizomes are present ex- 

 amination of these parts, after a suitable interval, will show that many 

 of their filaments of tracheae are injected. Meanwhile the parts above 

 the supplying leaf become coloured, and it will be seen that the distribu- 

 tion of coloured tracheae is decided by the anatomical connections of 

 those filaments of tracheae which directly convey the coloured liquid 

 from the point of supply through the petiole to the stem. In tracing 

 the path of the solution one is impressed with the fact that the path 

 of least resistance is by no means always the shortest path in the wood. 

 Transverse motion across several tracheae seldom occurs, and the 

 separate linear series of conducting tracheae are practically isolated from 

 each other laterally. Here we may recall Strasburger's experiments 

 showing the very great resistance offered to the flow of water in a 

 transverse direction in the wood of trees. This isolation of the separate 

 filaments of tracheae in the leaf and in the stem enables the tension 

 developed by the transpiring cells of the leaves, while it raises a column 

 of water in one series of tracheae, to draw down a solution in a neigh- 

 bouring filament of tracheae terminating above in some local supply. If 

 the anatomical connection of the two series is located in a subterranean 

 organ the tracheae of the subterranean organ may become filled from 

 that supply. 



So far the evidence of reversed flow in the water-conducting tracts 

 which we have been considering has been derived from plants under 

 artificial conditions- — plants whose conducting tracts have been cut into 

 and othei'wise interfered with. Is there any evidence that reversal of 

 the transpiration-curi'ent noi*mally occurs in uninjured plants? 



Some recent work on the transmission of stimuli seems to me to 

 indicate that these reversals are continually occurring in normally 

 growing plants. 



The first piece of work to which I would direct your attention is that 

 of Eioca on Mimosa. It has long been known that the stimulus which 

 causes the folding of the pinnules and the bending of the petioles of 

 Mimosa could traverse poi'tions of the petioles or stems which had 

 been raised to such a temperature as would kill the living elements in 

 these organs. Notwithstanding that observation, Haberlandt's view, 

 that the stimulus is transmitted as a wave of pressure through certain 

 tubular elements of the bast, was generally accepted as the least objec- 

 tionable of any of the theories which had been put forward to explain this 

 transmission. Ricca saw that, among other difficulties, the slowness 

 of transmission — never more than 15mm. per second — was a grave 

 objection to this view. Accordingly working with a woody species of 

 Mimosa — Mimosa Spegazzinii — he removed the whole bast and outer 



