Newcombe, Thigmotropism of terrestrial roots. t9 



was sealecl a glass-tnbe as describecl above. The lengtlis of tlie 

 collodion tubes below tlieir union with the glass-tubes were re- 

 spectively 20 mm, 25 mm, and 18 mm. The rate of fihration 

 of water through tlie collodion tubes was determined wlien the 

 whole of the collodion was below the surface of the snrronnding 

 water. After gradiiating the glass-tubes it was fomid that when 

 water was kept in these tubes to a height of not more than 

 11 cm nor less than 10 cm above the surface of the water be- 

 low, the collodion tubes filtered respectively at the rate of ,05 cc, 

 ,05 cc, and ,02 cc per minute. If we reduce these values to the 

 rate per Square millimeter of surface of membrane, we find that 

 they are -^/suo cc, ^/47i2 cc, and ^/voes cc or from ^3 to ^/t cubic 

 millimeter per minute. Consider now the possible rate of cur- 

 rent through the collodion tnl)os as they envelop the roots in a 

 revolving basin of water. The greatest velocity of current 

 employed in the following experiments was 448 cm per 

 minute, corresponding approximately to a head of 3,4 cm 

 of water. It is evident therefore from this and addition- 

 al considerations that filtration through the two sides 

 of the collodion tubes while on the roots must be but a 

 small fraction of that found in the tests where the head em- 

 ployed was 10 cm. By reference to the work of Juel^) and 

 myself -) it will l^e seen that velocities below 1 cm per minute 

 cease to call forth typical rheotropic responses in several species. 

 One species so tested was tlie same as that used in the present 

 experiments — Blmphanus sativus. The rate of flow through 

 one thickness of collodion membrane with a head of 10 centi- 

 meters having been found to be but one thirtieth of that not- 

 ed above as giving very poor responses, or none at all, in sev- 

 eral sjDecies, it is not to be supposed that the still much more 

 slowly flowing filtration streani through the collodion tubes while 

 on the roots could induce responses. We may therefore with 

 safety rely on any results coming from roots enveloped in these 

 collodion tubes, feeling assured that any cm-ves showing 

 themselves have not come from a filtration stream through 

 the tube. 



The seedlings for these experiments were carefully selected 

 in Order to use only those with straight roots, and were secured 

 by the lower parts of their hypocotyls to bars of white pine, the 

 bar being 20 cm long, and extending across the diameter of a 

 glass-basin of water. The collodion tubes were grasped at their 

 open ends by fine forceps, and were easily passed over the roots, 

 to which they readily ding when the basin of water is revolv- 

 ed. As the roots remain stationary in the moving water, the 

 collodion tubes in successful preparations are pressed closely 

 against the roots on the stream side and are out of contact wilh 

 the roots on tlio ojiposite side. Some of the collodion tubes 



1) Juel, Jahrb. f. wiss. Botan. XXXIV. 507. 



2) Newcombe, Botanical Gazette XXXIII. 266. 



